One hundred thirty-two patients with various connective tissue disorders, including 60 with rheumatoid arthritis (RA), had antibodies against human as well as Escherichia coli hsp60 in titers significantly higher than those of normal controls. There was a correlation between titers of antibody to human hsp60 and those to E. coli hsp60. Levels of antibodies against human and E. coli hsp60 were lower in joint fluids than in sera, indicating little production of antibodies in the joint. Antibodies affinity-purified with E. coli hsp60 bound strongly with the homologous hsp60, but weakly with human hsp60. However, antibodies affinity-purified with human hsp60 bound comparably with both E. coli hsp60 and human hsp60. Antibodies affinity-purified with Mycobacterium tuberculosis hsp65 bound to human hsp60 with a reactivity similar to the reactivity of those affinity-purified with human hsp60. The reactivity to the three hsp60 species was lost when sera were absorbed with E. coli hsp60, while the reactivity to E. coli hsp60 remained after extensive absorption with M. tuberculosis hsp65 or human hsp60. These results indicate that anti-hsp60 antibodies in patients with RA and other connective tissue disorders are raised by infection with intestinal microorganisms such as E. coli. They may represent another example of autoimmune responses triggered by antigenic mimicry of host proteins to microbes and suggest that the reactivity of antibodies from RA patients with M. tuberculosis hsp65 might have been a cross-reaction with the E. coli homologue.