A metabolic-inhibition test was used to investigate the growth-inhibiting properties of various antisera to Mycoplasma mycoides. Bacteriocidal and bacteriostatic titres were demonstrated. The former were dependent on the initial mycoplasma inoculum, whereas the latter were not. Both titres were potentiated by complement, but the degree of complement dependence varied from one antiserum to another and this may explain the present divergence of opinions on the role played by complement in growth-inhibition. © 1969.