An established in vitro cell line (LLTC), originally derived from the Lewis lung carcinoma (LL), was found to have lost sensitivity to the C-nucleoside antitumor agent tiazofurin (NSC-286193; 2-beta-D-ribofuranosylthiazole-4-carboxamide) both in vitro and in vivo. A new in vitro cell line (LLAK) was derived from LL and compared to LLTC in its growth properties and sensitivity to tiazofurin. LLAK resembled the in vivo tumor in having both a high S-phase fraction and a high rate of cell death at high cell density. In continuous drug exposure growth inhibition assays, the concentration of tiazofurin required to reduce the number of cells in a culture by 50% with respect to control cultures was 0.51 microM for LLAK, 2.6 microM for LLTC, and greater than 10 microM for a range of human cancer cell lines. In cytotoxicity assays involving a 2-hour drug exposure followed by clonogenic assay, tiazofurin was more toxic to LLAK cells than to LLTC cells or L1210 murine leukemia cells, consistent with its high in vivo activity against LL. MM-96 human melanoma cells were highly resistant. Flow cytometry studies indicated that tiazofurin selectively depleted the LLAK cell population of S- and G2-phase cells. In one experiment involving 16 consecutive in vitro passages of LLAK in the absence of tiazofurin, a new line emerged that was resistant to tiazofurin in the clonogenic assay. The results demonstrate the spontaneous emergence of resistance from a tiazofurin-sensitive cell line. If similar processes occur during adaptation of human tumor cells to culture, this may explain the finding of low activity of tiazofurin toward a range of human tumor cell lines.