A 10 mg dose of Cyclophosphamide was administered intraperitoneally to guinea pigs for 5 days at various stages of sensitization, and the effects of the drug for allergic contact dermatitis were studied by vascular permeability. This permeability was markedly increased in the group that was sensitized after administration of Cyclophosphamide, but the condition was inhibited in the group given the drug after sensitization but prior to provocation. Histopathological observations were made using indices of epidermic thickening (hypertrophy) and number of epidermic infiltrating cells. A more severe dermatitis was noted in the group given Cyclophosphamide prior to sensitization, as epidermic thickening in the animals in which the lesser concentration of DNCB had been applied, while there was an infiltration of cells in animals to which a higher concentration of DNCB had been given.