17 beta-acetamido-3-aza-homo-4 alpha-androsten-4-one has cytostatic activity against Ehrlich ascites tumor, L1210, and P388 leukemias in mice when administered intraperitoneally. The effect of the homo-aza-steroid on the incorporation of radioactive precursors to DNA and RNA of L1210 leukemia cells was investigated. It was found that treatment of cells with 12.5 micrograms/ml of the drug for 1 hour inhibited DNA synthesis by 81%. This was partly because the drug affected the radioactive thymidine pool in the cell. The inhibitory effect was found to be reversable. The incorporation of [3H]thymidine into DNA was lower when cells were incubated in the presence of S9 mix. It was also found that the same compound inhibited RNA synthesis by 67%.