The kinetic effect of cyclophosphamide (CY) was investigated using the immune response to sheep red blood cells (SRBC), activation of the mononuclear macrophage system and altered susceptibity to pseudomonas infection. The level of delayed type hypersensitivity (DTH) was enhanced with suppressed antibody formation, when antigenic stimulation was given about 3 days after CY administration. In contrast, antibody formation increased markedly when challenged with antigen about 10 days after CY administration. Activity of macrophage system as measured by rate of carbon clearance and spreading of peritoneal macrophage was decreased within 6 days after CY, therefore increased to a peak at the 13th day of CY administration. CY increased a susceptibility to pseudomonas infection at the early time of its administration such as the 3rd day, whereas more increased resistance was observed at the later time such as the 13th day. These results indicated that B-lymphocyte depletion included by administration of sublethal dose of CY (200mg/kg) was followed by vigorous hyperplasia of B-lymphocyte.