Sera of persons without known actinomycotic infection (n = 153) were tested for antibodies reacting with antigenic preparations of different Actinomyces spp. and Nocardia spp. By using an enzyme immunoassay, 16% of all of the sera analyzed reacted significantly with antigens of A. viscosus and 2% those of with A. naeslundii. The antigens detected by these antibodies were of low molecular mass (14-32 kDa) and showed a uniform reaction pattern in the immunoblot analysis. Multiple bands coccurred with a difference of approximately 2 kDa in size suggesting the presence of repetitive units. Analogous antibodies were not observed with A. israelii serovar 1 and serovar 2 antigens, which were only insignificantly bound by the sera. The antibodies demonstrated were most probably acquired during episodes of periodontal disease or gingivitis in which A. viscosus is etiologically involved. Antibodies against nocardial antigens were not demonstrated in significant proportions by enzyme immunoassay and immunoblot analysis.