An experimental immunostimulating complex vaccine has been prepared from detergent (Mega-10) solubilized Mycoplasma gallisepticum (MG) antigens. Sucrose gradient centrifugation, SDS-PAGE and immunoblotting studies demonstrated that the ISCOM vaccine contained virtually all of the immunodominant MG membrane proteins, including p64 and p56. Protective immunity generated by the experimental MG ISCOM vaccine was demonstrated in challenge experiments. Chickens immunized with a single dose containing between 1 and 50 micrograms of MG ISCOMs had significantly reduced lesion scores in the air sac after challenge. The reisolation of the challenge MG strain was significantly less frequent from the chickens in vaccinated groups than from the unvaccinated control group. Presence of humoral antibodies in chickens vaccinated with 1-25 micrograms MG ISCOMs was not detectable by blocking ELISA or haemagglutination-inhibition before challenge. Chickens vaccinated once or twice with a 50 micrograms dose were transitory positive by blocking ELISA and rapid plate agglutination before challenge.