Sera from chickens inoculated with various challenge infectious bursal disease viruses or infectious bursal disease vaccines were found to cross-react in the Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS) serum plate agglutination (SPA) tests. Two-fold dilutions of these cross-reacting sera with phosphate-buffered saline before retesting eliminated all non-specific agglutination in the MG and MS SPA tests. Cross-reactions were observed in the SPA test using sera from chickens inoculated with either MG or MS. Dilutions of these sera 1:2 had little effect on the number of these cross-reactions. At 1:4 serum dilutions, however, the number of cross-reactions between MG and MS was reduced. At 1:8 dilution of test sera, cross-reactions between MG and MS were further reduced. Some reduction in specific MG and MS SPA reactions, however, also occurred at the 1:8 dilution of sera with some of the plate antigen.