The liquid phase C1q[q fragment of complement component 1]-binding assay was used as a measure of circulating immune complexes in 147 renal transplant recipients. Patients (36) were studied serially from the time of transplantation. Abnormal levels of C1q-binding activity (C1qBA) were most frequently detected in the first 6 mo. post-transplant. Although there was a statistically significant association of elevated C1qBA with rejection, the incidence of false positives and false negatives was high and particularly evident in serial studies. There was no evidence of association between C1qBA and the recipients'' original renal diseases. Post-transplant monitoring of renal transplant recipients for circulating immune complexes by the C1q-binding assay is an unreliable guide to rejection.