The toxicity of cocaine and crack was studied on the protozoan Tetrahymena pyriformis, using several endpoints, such as the DNA content of the macronuclei and the phagocytic ability. Both forms induced an increase in the DNA content of the protozoan, which indicates the stimulation of the mitotic process. In contrast, the phagocytic activity, of the protozoan was decreased after the administration of cocaine, an effect that was more extensive after the administration of crack. These results, derived from previous experiments, suggest a possible relationship between the observed immunosuppression in cocaine abusers and the immunosuppression found in the protozoan. This suppression subsequently may play a role in the development of other opportunistic infections in drug abusers. This paper, based on in vivo experiments with the protozoan Tetrahymena, suggests the compromised immune response in cocaine addicts and assures the reported effects of cocaine on immune cell function.