Reactivity of Mycoplasma pulmonis (Mp) antigen strains to anti-Mp antibody in the sera of Mp infected rats was examined by enzyme linked immunosorbent assay (ELISA). Antibody titers to 7 kinds of Mp antigens were measured in the sera of 20 Mp isolated rats and 20 Mp free rats by ELISA and complement fixation test (CF test). ELISA showed that there was no difference in the antibody titer of the same serum among 7 Mp antigen strains employed, and the main cross reaction to anti-Mp antibody took place on the common recognition site (common antigen) in all the Mp antigens. The CF test suggested that the antibody titers largely differed due to the kind of Mp antigen strains, and the detection rate was between 0 and 60%, presumably due to the difference in the reactivity or binding ability of complements but not due to the difference in the cross reactivity of Mp antigen to anti-Mp antibody. When the cross reactivity of anti-Mp antibody to Mycoplasma arthritidis (Ma) antigen was examined in the sera of 33 Mp isolated rats, the CF test exhibited the negative results, but ELISA showed 4 positive cases to support the cross reactivity of anti-Mp antibody to Ma antigen.