The genetic activity of cyclophosphamide (Cy) was tested in the host-mediated assay (injection of yeasts into the peritoneal cavity of rats) modified by the use of probenecid (Pro) (Benemid) to block the renal tubular excretion of the genetically active metabolites. The genetic test system used was the induction of mitotic gene conversion in two unliked loci of a diploid strain of Saccharomyces cerevisiae. By this method the genetic effect of By was doubled in comparison with the case of administering Cy alone. Compared with the animals which received only Pro, increases of conversion frequencies of 20 times in the ade2 locus and of 15 times in the trp5 locus were found.