The phagocytic function of the reticuloendothelial system (RES) in the rat has been studied after the administration of cyclophosphamide, 5-fluorouracil or a combination of these drugs and vincristine. No impairment in the elimination of 125I-labelled micro-aggregated human serum albumin was found. A significant reduction in relative and total spleen weight was not paralleled by a decreased splenic uptake of the RES test substance per gram tissue. A significant increase in the relative weight of the lungs was paralleled by a significant increase in the uptake of test substance on a per gram basis. No corresponding measurable effect was found in the liver. A general reduction in the cell population of the haematopoietic system has to be assumed. If this reduction is more pronounced in sub-populations of T lymphocytes (suppressor T cells) has to be further tested.