The objective of this study was to develop a rapid in vitro method for predicting the response of human tumors to anticancer drugs. In this study an attempt was made to correlate the drug effects on the relative incorporation of (3H) thymidine (ThdR) into DNA with the sensitivity of tumor cells to that drug. The results of the study indicate that following treatment of the cells with adriamycin (ADR) or 1-(2-chloroethyl)-3-(4-methyl cyclohexyl)-1 nitrosourea (MeCCNU), there was a significant inhibition of DNA synthesis in the drug-sensitive cells. However, the inhibition was relatively small in the drug-resistant cells. Following cytosine arabinoside (Ara-C) treatment, a dramatic recovery in the rate of DNA synthesis was seen in Ara-C-resistant cells but not in cells sensitive to Ara-C. Thus, the method described in this study appears to be capable of distinguishing whether a tumor cell line is sensitive or resistant to a given drug.