The suppressive effect of Mycoplasma gallisepticum (MG) on Haemophilus gallinarum (HG) immune response was shown. Antibody response to HG was highly suppressed when chickens were inoculated intramuscularly with HG-MG combined bacterin. Findings were similar in chickens injected intramuscularly with HG and MG bacterin separately at adjacent sites. No immunosuppressive effect was recognized when injections with HG and MG bacterins were in the left and right thigh muscles, respectively, or from intravenous inoculation with the combined bacterin. Nor did HG-Mycoplasma synoviae (MS) combined bacterin injected intramuscularly evidence immunosuppression. Recovery rate of HG and clinical symptoms were more evident in chickens with suppression of antibody responses than in chickens without suppression.