The effect of cisplatin, carboplatin, oxoplatin, and iproplatin on delayed-type hypersensitivity in mice was studied. Comparing the equitoxic doses, which were close to therapeutic dosages, we found a greater inhibition of delayed-type hypersensitivity in the second-generation platinum complexes than in cisplatin. This inhibition occurred mainly when these drugs were applied after sensitization. This phenomenon might be explained by the antimitotic action of platinum cytostatics. The lower toxicity of second-generation platinum drugs was not accompanied by a corresponding decrease of immunotoxicity.