The occurrence of circulating immune complexes was investigated in 31 patients with cytomegalovirus infection (29 infected in utero and 2 with natal infection) and 34 uninfected controls. Anti-complementary activity above 1:20 occurred in 34% (29/86) of the sera tested from the infected group in contrast to 7.5% (3/40) in the controls (P < 0.005). When assayed by means of a lymphoblastoid cell line (Raji cell test), the reactivity in these groups was 45 (39/86) and 2.7% (1/36), respectively (P < 0.001). Correlation of results between these two complement-dependent assays occurred in 75% of samples collected from the infected group. Frequency of reactivity was higher in severe intrauterine infection and during the 1st yr of life paralleling the patterns of viral excretion and humoral immune responses. Physicochemical characterization demonstrated that reactive substances in sera were acid-dissociable and, in one sample tested, contained 7S IgG antibodies with cytomegalovirus (CMV) specificity. Circulating immune complexes were heavier (18-22S) in sick, as opposed to subclinically CMV-infected patients, in whom intermediate size complexes (12-16S) were found. In three of four symptomatic patients whose demise was due to severe congenital infection, granular deposits of immunoglobulins and C3 were detected in a pattern typical of immune complexes along the glomerular basal membrane of the glomeruli. Whether or not circulation and deposition of heavier immune complexes contributed to the adverse clinical outcome is unresolved. Because of the high incidence of both congenital and natal CMV infections, definition of the pathogenetic potentials of both heavy and intermediate size immune complexes is required to design more effective therapeutic measures.