The relation between the duration of bacterial infection and circulating immune complexes (CIC) level was evaluated using the C1q binding assay in a group of patients with well defined clinical sepsis. Fifty-four patients with endocarditis and 35 with post-open heart surgery mediastinitis were prospectively studied over a period of 2 years. CIC were detected in 42% of patients studied. Interindividual variations were observed but it was found that the level of CIC increased statistically with time (P less than 0.001). CIC were statistically linked with cryoglobulinemia (P less than 0.001), rheumatoid factor (P less than 0.001) and a decreased CH50 (P less than 0.05). CIC were more frequent in patients with endocarditis (53%) than in patients with mediastinitis (24%). However, when the duration of the infection was taken into account the difference was no longer significant. No relation could be evidenced between the incidence of CIC and clinical symptoms including prognosis and renal signs. In our experience, determination of CIC does not have a critical clinical value.