Rapamycin exhibits activity against several ascites and solid transplantable tumors; it is slightly active to inactive against leukemias. On a weight basis, rapamycin was less active than 5-fluorouracil, cyclophosphamide and adriamycin, but rapamycin's maximal activity against Colon 38 tumor was similar to that of 5-fluorouracil and cyclophosphamide. Its activity was such that it significantly inhibited tumor growth at any stage of development. In the active dose range, rapamycin appeared less toxic than the other drugs. In the Colon 38 tumor model, rapamycin at a given dose exhibited the same activity when administered ip, iv, im and sc; upon oral administration, its activity was reduced but not abolished. Rapamycin was compatible with 5-fluorouracil and cyclophosphamide. The sequential treatment 5-fluorouracil-rapamycin-cyclophosphamide was superior to the sequence 5-fluorouracil-adriamycin-cyclophosphamide in protecting Colon 38 tumor-bearing mice. 29-Demethoxyrapamycin exerted only marginal activity against P388 lymphocytic leukemia; it was inactive against B16 melanocarcinoma and Colon 38 solid tumor.