Cyclophosphamide was given intraperitoneally to guinea pigs in a dose of 10 mg/day for a period of 5 days and the histological changes were studied on the 1st,5th and 10th days after the administration. Changes in number of pyroninophilic cells due to DNCB sensitization were also studied. On the 1st day after discontinuation of Cyclophosphamide, the germinal centres, lymph follicles and paracortical areas of the lymph nodes all showed the same degree of cell component loss, but on the 10th day, only the germinal centres had been restored to their normal level, while the other sites showed a delay in recovery. The administration of Cyclophosphamide prior to DNCB sensitization resulted in a greater increase in pyroninophilic cells in the paracortical area of the regional lymph nodes than in the controls, between 7 to 13 days after sensitization. No increase was noted in the group that was given the drug after sensitization.Thus decrease in the lymph node component was accounted for by the B-cells which are susceptible to the effects of Cyclophosphamide, and the increase in pyroninophilic cells was attributed to this marked decrease in B-cells.