Experiments are reported in detail which show that an antibody which appears in the blood of certain rabbits implanted with the Brown-Pearce tumor or injected with cell-free extracts of it is capable of suppressing the growth of the tumor cells under a variety of experimental conditions, the effects of the antibody being wholly distinct from those of unknown factors that frequently bring about regression of the growth. The implications of the findings are discussed with particular reference to facts indicating that the distinctive cell constituent with which the antibody reacts may play a significant part in the proliferative activities of the Brown-Pearce tumor cell.