The sera of patients with dermatitis herpetiformis, Henoch-Schönlein purpura and systemic lupus erythematosus were examined for IgA-containing immune complexes using a newly described radioimmunoassay. The IgG Raji cell radioimmunoassay and the 125I-C1q binding assay were also used to detect IgG- and IgM- containing soluble immune complexes. IgA-containing immune complexes were found in the sera of twelve of forty-nine (24%) patients with dermatitis herpetiformis, four of six (67%) patients with Henoch-Schönlein purpura, and seven of ten (70%) patients with systemic lupus erythematosus. IgG- or IgM- containing immune complexes were also found in six of forty-seven patients with dermatitis herpetiformis, in one of six patients with Henoch-Schönlein purpura, and in nine of ten patients with systemic lupus erythematosus, by either the 125I-Clq binding assay or the IgG Raji cell assay. The finding of soluble IgA immune complexes in a high percentage of patients with systemic lupus erythematosus and Henoch-Schönlein purpura suggests that they may play an important role in the pathogenesis of these diseases. In contrast, their low prevalence in patients with dermatitis herpetiformis suggests that IgA-containing immune complexes may not play a major role in the pathogenesis of dermatitis herpetiformis.