The chemotherapeutic action of tegafur (FT) against adenocarcinoma 755 in mice was markedly potentiated by oral administration of L-cysteine and L-cystine without increasing its toxicity. In particular, the combination of FT at 200 mg/kg per day (maximum dose) and 1000 mg/kg per day of L-cystine markedly inhibited tumor growth. The dose ratio of L-cysteine or L-cystine to FT needs 5 by weight to potentiate the antitumor activity of FT. The antitumor activity of 5-fluorouracil (FU) was slightly, but not significantly, increased by L-cysteine. The total concentration of FT in the plasma and the tumor when it was given in combination with L-cystine was significantly increased when compared with FT alone 1 h after oral administration. The FU level in the plasma after administration of the combination of FT and L-cystine was three times higher than that after FT alone, and the FU level in the tumor after treatment with the combination of FT and L-cystine was also higher (about 20%) than that after FT alone. This significant increase in FT and FU levels in the plasma and the tumor may be related to the potentiation of the antitumor activity of FT by L-cystine.