The reaction of spontaneously occurring human anti-human antibodies and experimentally produced rabbit anti-actin antibodies was investigated in a solid-phase radioimmunoassay (RIA). Three structurally different in vitro forms of actin monomeric G-actin filamentous F-actin and aggregated denatured actin were used as antigens. Human anti-actin antibodies reacted with F- and G-actin but not with aggregated actin, while rabbit anti-actin antibodies gave a strong reaction with all 3 forms of actin indicating differences in antibody specificities. The results of the anti-actin RIA were compared with those obtained by indirect immunofluorescence (IFL) on cryostat sections of rat stomach. The anti-actin RIA discriminated between patients' sera and control sera in most cases, although the indirect IFL test gave more conclusive results. The seemingly low sensitivity of the anti-actin RIA compared with that of indirect IFL test for detection of human anti-actin antibodies is probably due to favourable antigen distribution in tissue, not available in the solid phase. The anti-actin RIA was able to detect anti-actin antibodies in 8 out of 8 immunized rabbits although only two produced antibodies detectable by indirect IFL. The differences in reactivity between the two methods may depend on the presence of aggregated denatured actin in the antigen preparation used for immunization and exposure of the corresponding antigenic determinants of actin on the solid phase.