We compared the in vitro sensitivity patterns to cytotoxic drugs and expression of the multidrug resistance-associated MDR1 gene (also known as PGY1 gene) in four gastric carcinoma cell lines with those obtained in a panel of 11 colorectal carcinoma cell lines. In addition, we tested the effects of leucovorin on enhancement of fluorinated pyrimidine-induced cytotoxicity. We used a semiautomated tetrazolium dye assay [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (tetrazolyl blue)] (MTT) to compare the drug sensitivity of the gastric carcinoma cell lines with that of the colorectal carcinoma cell lines. The gastric carcinoma cell lines were more sensitive to some drugs, including doxorubicin and cisplatin, but not to the fluorinated pyrimidines. Addition of leucovorin at a clinically achievable concentration enhanced the cytotoxic effects of both fluorouracil and floxuridine in colorectal carcinoma cell lines, but it enhanced the effects of only floxuridine in gastric carcinoma cell lines. With the use of a slot blot assay, relatively low levels of MDR1 RNA were present in all four gastric carcinoma cell lines, while intermediate or high levels were present in most of the colorectal carcinoma cell lines. In general, our findings reflect clinical experience and may help in the design of clinical trials.