The in-vitro growth inhibition of cancer and normal cell lines caused by mixed or covalently linked antimetabolites should clarify whether the conjugation of antimetabolites influences cell sensitivity and growth inhibition in a manner that differs from an equimolar mixture of the same antimetabolites or not. Growth inhibition of the human gastric adenocarcinoma cell lines 23132/87 and MKN-45 in comparison with normal gastric intestinal CCL-241 and the dermal fibroblast cell line NHDF was evaluated using CASY technology. The cell lines were incubated with an equimolar mixture of 5-fluoro-2′-deoxyuridine (5FdU)+3′-C-ethynylcytidine (ECyd) or the covalently linked duplex drug 5FdU(5′→5′)ECyd. The drug and metabolites of the assays and medium were determined semiquantitatively using high-performance liquid chromatography. The sensitivity of cancer and nonmalignant cell lines was clearly different against the duplex drug. A measure of 0.65 µmol/l 5FdU(5′→5′)ECyd, for example, reduced the growth of MKN-45 or 23132/87 gastric cancer cells from 100% on day 0 to about 50 or 20% on day 10, respectively. However, under the same conditions, the growth of the nonmalignant NHDF and CCL-241 cell lines was not markedly inhibited. The cytostatic activity of the duplex drug is based on the active metabolites in and outside the cell formed by the degradation of 5FdU(5′→5′)ECyd. The sensitivity of cell lines against the duplex drug depended on its ability to metabolize the duplex drug. 5FdU(5′→5′)ECyd should be more advantageous for specific and efficient polychemotherapy of gastric cancer than the corresponding equimolar mixture of 5FdU+ECyd or a standard combination regime of single drugs.