The influence of three chosen model sulfhydryl groups containing compounds, characterized by increasing size of molecule--cysteine, glutathione and insulin on antineoplastic activity of 5-fluorouracil (5-FU) and 6-mercaptopurine (6-MP) against murine leukemia L-1210 5-FU and 6-MP and murine sarcoma Sa 180, was studied. Sulfhydryl compounds failed to change in most cases the therapeutic (antineoplastic) effect of 5-FU and 6-MP. However, certain doses of these sulfhydryl compounds enhanced the cytostatic effect of 6-MP in respect to Sa-180.