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CO-111 Denial Code: Assignment Must Be Accepted

CO-111 means the service is not covered unless the provider accepts assignment. The claim was submitted as unassigned (or assignment was not indicated), but the payer only pays this service when the provider agrees to accept the approved amount as payment in full.

Code: CO-111 (CARC 111)
Status: Active In use since January 1, 1995.
Code set: Claim Adjustment Reason Codes (CARC)
Group codes: CO (Contractual Obligation): The provider is responsible for the assignment error. Because assignment is required, the patient generally cannot be billed for the amount.
Official description: Not covered unless the provider accepts assignment.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What CO-111 means

CARC 111 says the service is not covered unless the provider accepts assignment. Assignment is the provider’s agreement to accept the payer’s allowed amount as full payment, collecting only deductible, coinsurance, and co-payment from the patient. Some services are payable only on an assigned basis. When those services arrive on an unassigned claim, the payer denies them with CARC 111.

This comes up most with Medicare, where non-participating providers can choose assignment claim by claim, except for services where assignment is mandatory. Other payers may have similar rules.

Example: a non-participating provider submits a claim that includes a clinical lab test and marks box 27 “No.” The payer denies the lab line with CO-111 and remark N71.

Common causes

Box 27 (accept assignment) marked “No” on a claim containing services that require assignment.
A billing system default set to unassigned for non-participating claims, applied to every line.
The 837P assignment or plan participation indicator missing or wrong.
A provider’s participation status changed and claim settings were not updated.

How to fix it

Confirm the service requires assignment under the payer’s rules.
Correct the assignment indicator (box 27 or the 837 field) to accept assignment.
Resubmit. Depending on whether the payer rejected it or processed it, send a new claim or a corrected claim with resubmission code 7 in box 22. Some Medicare contractors handle this through reopening; follow your MAC’s instructions.
Recalculate the patient balance. On an assigned claim, the patient owes only cost-sharing, not the difference between your charge and the allowed amount.
If the claim was marked correctly, contact the payer and request reprocessing.

How to prevent it

Set claim defaults by service type so lines that require assignment are always submitted as assigned.
Review your Medicare participation status and billing settings each year.
Add a scrubber rule that flags unassigned claims containing mandatory-assignment services. A Claims Validator can enforce it.
Train staff on the difference between participating, non-participating with assignment, and unassigned claims.
For related enrollment settings that affect payment, see provider enrollment denials.

Remark codes that may appear with CO-111

N71 (Your unassigned claim for a drug or biological, clinical diagnostic laboratory services or ambulance service was processed as an assigned claim.): Medicare remark explaining that an unassigned claim for a service requiring assignment could not be paid.
N777 (Missing Assignment of Benefits Indicator.): The assignment of benefits indicator is missing.
MA09 (Alert: Claim submitted as unassigned but processed as assigned in accordance with our current assignment/participation agreement.): The opposite outcome: the claim was submitted unassigned but processed as assigned under program rules.

CO-100 (Payment made to patient/insured/responsible party.): Payment made to the patient instead of the provider, another assignment-related result.
CO-16 (Claim/service lacks information or has submission/billing error(s).): General missing or invalid information, sometimes used for a missing assignment indicator.
CO-B7 (This provider was not certified/eligible to be paid for this procedure/service on this date of service.): Provider was not certified or eligible to be paid for the service on that date.

CO-111 FAQ

What does accepting assignment mean?

It means the provider agrees to accept the payer's approved amount as full payment and collect only the patient's cost-sharing. For Medicare, participating providers accept assignment on all claims.

Which services require assignment?

It depends on the payer. Medicare, for example, requires assignment for certain services, such as clinical diagnostic laboratory tests and services of some non-physician practitioners. Check the payer's rules for your service.

How do I fix CO-111?

Mark accept assignment (box 27 on the CMS-1500 or the corresponding 837P indicator) and resubmit according to the payer's instructions.

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