Thirteen human lung cancer cell lines, 7 representing small cell lung cancer (SCLC) and 6 different types of non-SCLC, were tested for sensitivity to tumour necrosis factor alpha (TNF-alpha) and interferon alpha and gamma (IFN-alpha and gamma) using an automated fluorometric microculture cytotoxicity assay (FMCA). One SCLC line (H-82) was found to be sensitive to IFN-alpha in short-term (72 h) culture, whereas after prolonged (5 days) culture two additional SCLC cell lines responded to IFN-gamma. TNF-alpha inhibited the growth of one large cell carcinoma cell line (H-157), whereas all SCLC lines were found to be insensitive. The combination of IFN-gamma and TNF-alpha produced no further response compared with the single agents used alone. By continuous cultivation of the IFN-alpha-sensitive cell line H-82 in the presence of increasing concentrations of IFN-alpha, an IFN-alpha-resistant subline (H-82) was established. This line displayed a high degree of resistance ( > 100 fold) to IFN-alpha and cross-resistance to IFN-gamma. There was no alteration in the number of IFN binding sites, in the growth rate, the expression of selected surface markers for SCLC or the expression of multidrug resistance markers in the H-82R subline compared with the parental H-82 cell line. The results demonstrate a heterogeneous response of SCLC cell lines to IFN-alpha and gamma and TNF-alpha with only a minority of the cell lines responding to these agents by growth inhibition. The IFN-alpha and gamma H-82R subline may serve as a valuable tool in future studies on the mechanisms of IFN antitumour activity.