We have investigated IgG antibodies to microfilament-associated proteins in sera from 29 normal controls and from 172 patients with autoimmune diseases such as rheumatoid arthritis (n = 45), systemic lupus erythematosus (n = 43), polymyositis/dermatomyositis (n = 36), systemic sclerosis (n = 35), and autoimmune chronic active hepatitis (CAH, n = 13). We observed, by indirect immunofluorescence, a staining pattern in which the fluorescence was continuously distributed along microfilaments by using several animal and human cell lines as substrate. Because no reactivity with individual bands or specific combinations of bands was observed by immunoblotting using different cell lines, we further characterized our sera by immunoblotting using a panel of the purified microfilament-associated proteins filamin, myosin, alpha-actinin, actin, tropomyosin, and myosin light chain. Results shows that normal as well as autoimmune sera are polyreactive with microfilament proteins. The specificity of reactivity against a particular microfilament-associated protein was demonstrated by inhibition experiments with the specific protein. No correlation was observed between immunoblot and immunofluorescence results. Of the 201 sera, 40 (20%), 42 (21%), 31 (15%), 26 (13%), 34 (17%), 24 (12%), 4 (2%), and 0 (0%) reacted, respectively, by immunoblotting with 0, 1, 2, 3, 4, 5, 6, or 7 of the microfilament-associated proteins. Furthermore, 57 different profiles of reactivity with the panel proteins were observed. All autoimmune CAH sera reacted with at least 3 proteins. This result was strikingly unique to this group. Anti-actin antibodies were as common in normal as in CAH sera. Anti-alpha-actinin antibodies were strikingly more frequent in CAH than in any group. We conclude that a remarkable diversity of anti-microfilament antibodies is present in normal as well as in autoimmune sera and that a continuous microfilament fluorescent pattern is not restricted simply to anti-actin antibodies. Furthermore, sera negative by immunofluorescence react with several miocrofilament-associated proteins by immunoblotting. Reactivity with multiple (> or = 3) microfilament-associated proteins including alpha-actinin distinguishes CAH sera from normal and other autoimmune sera.