SUMMARY Chicken antisera to the s6 strain of Mycoplasma gallisepticum were used to study the mechanism of metabolic inhibition and growth inhibition by specific antibody. In high dilutions of antiserum the lag phase of growth was significantly prolonged, but eventually growth commenced and proceeded normally. In low dilutions of antiserum, growth was permanently inhibited. The prolongation of the lag phase was greater in a medium containing 10 yo horse serum than in 10 yo swine serum; with chicken serum in the medium the lag phase was of normal duration. The results correlated with the observation that the metabolic inhibition (m.i.) titres of antisera were higher when the test was conducted in horse serum than in swine or chicken serum. The antigens on the organism associated with the inhibition of growth by antiserum appeared to be essential physiologically active ' receptors '. Antisera to growth medium caused agglutination of the organism, but did not in- hibit growth or metabolism.