1 The effects of the H2-receptor antagonist, metiamide, on two types of cell-mediated immune response in vivo was investigated in the guinea-pig, in a test of the hypothesis that H2-receptor antagonists would result in 'runaway' unregulated hypersensitivity reactions. 2 Metiamide in doses of 5 mg-125 mg/kg given by 6-hourly injection during sensitization and challenge did not modify the delayed hypersensitivity reaction to tuberculin. 3 Metiamide in doses of 25-125 mg/kg given by injection, 6-hourly throughout sensitization to dinitrofluorobenzene (DNFB) and subsequent challenge six days later, did not modify the contact sensitivity reaction to DNFB. 4 It is concluded that metiamide, given during these cell-mediated immune reactions in the guinea-pig, does not enhance cell-mediated hypersensitivity responses.