The specificity of the in vivo humoral immune response elicited by procainamide was examined by solid-phase assays, immunofluorescence, immunoprecipitation and a cytotoxicity assay. Serial samples obtained from patients during their procainamide therapy showed a progressive increase in antibodies to histones and denatured DNA, and both activities decreased after discontinuation of therapy. In contrast antibodies to tetanus, human IgG (rheumatoid factor) and heterologous lymphocytes were unaffected by procainamide treatment, indicating that they were not drug-induced. Of 29 sera examined by protein-A-facilitated immunoprecipitation, four sera had antibody to ribosomal RNA and three sera immunoprecipitated a 40kD protein. Antinuclear antibodies were invariably present but absorption studies showed that these activities were due to anti-histone antibodies. These results indicate that procainamide-induced autoimmunity is characterized predominantly by an anti-histone and anti-denatured DNA immune response.