Using a C1q binding test, circulating immune complexes (IC) were detected in 33.3% of sera from 138 patients and in 19.4% of 124 spinal fluid samples from patients with multiple sclerosis. Most often they occur in sera alone. As a rule their detectable amount is small in sera as well as in spinal fluids. IC were observed with equal frequency during acute exacerbations and in stable phases of the disease. In patients with early MS of less than 3 months duration, IC were detected only rarely, whereas their frequency increased up to 50% in patients with longer standing disease. Immunosuppressive therapy has no influence on IC formation. Patients with immune complexes exhibited a more rapid clinical deterioration if compared as a group with IC-negativ ones. No correlations were found between immune complex formation and the CSF-IgG index or the rate of pleocytosis in spinal fluids. Neither the complement factors C3, C4, C3A nor total hemolytic complement activities (CH50) in serum were significantly decreased in patients with IC formation in serum as compared with the IC-negative group. The results demonstrate that IC formation probably is of no importance in the pathogenesis of multiple sclerosis.