Circulating immune complexes were measured in sequential monitoring studies in rats bearing chemically induced tumours at different body sites. The assay employed were based upon radioimmunoprecipitation of serum factors with 125I-C1q or with an indirect test whereby the inhibition of binding of 125I-C1q to IgG aggregates by tumour-bearer sera was measured. 125I-C1q-binding immune complexes were detected at the initial phase of tumour growth and the level of this activity returned to the normal range after tumour excision. With tumour growing at subcutaneous sites or in th peritoneal cavity, serum-borne C1q-binding material, after reaching a maximum level, decreased despite tumour progression and eventually fell into sub-normal ranges at terminal stages of growth. In contrast, following intravenous tumour cell injection, immune-complex levels increased until terminal stages of disease. The present findings indicate that the measurement of C1q-binding serum factors represents a useful method for monitoring the growth and burden of experimental animal tumours.