This paper reports the potentiation of the therapeutic effect of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) by amphotericin B (AMB) in s.c. transplanted murine ependymoblastoma 01B111. The rate of 2-month cures was 6% when tumors were treated 10 to 12 days after transplantation by a single 6 i.m. injection of 2.5 mg of CCNU per kg and reached 15% with 10 mg of CCNU per kg. When 25 mg of AMB per kg were given i.p. 10 hr prior to CCNU, the respective figures increased to 18 and 58%, the differences being significant at 5 and 1%. A single dose of 25 mg of AMB per kg given alone did not affect tumor growth. Radioactivity per g of tumor was measured 30, 60, and 120 min after injection of [14C]CCNU in total tissue, chloroform:methanol (2:1, v/v) extract, and CCNU isolated by thin-layer chromatography. No difference was found between animals treated with 25 mg of AMB per kg and controls. The inhibition of DNA synthesis, measured 24 hr after the administration of CCNU and 2 hr after the injection of [3H]thymidine, was almost optimal with 5 mg of CCNU per kg. The inhibition caused by 1 mg of CCNU per kg was not enhanced by AMB. Thus, the potentiation of CCNU by AMB does not seem attributable to an increased permeability of the tumor to CCNU or to an enhancement of the inhibition of DNA synthesis, at least in murine ependymoblastoma.