In vitro drug sensitivity testing (DST) of long-term cultures from small cell lung cancer (SCLC) tumours was correlated with response and survival after four cycles of etoposide and cisplatin. 27 cell lines from 25 patients were tested by the semi-automated MTT assay after a median culture of 29 months. The logs of the IC50 concentrations for etoposide and cisplatin were correlated with each other. For both drugs, median IC50 values of patients with partial or complete responses ("responders") were significantly lower (7-8 fold) than those of non-responders. When survival was plotted according to whether drug IC50 values were in the upper or lower halves, curves for etoposide were significantly different, but those of cisplatin were not. DST of the long-term cell lines by MTT assay was significantly correlated with the Weisenthal dye exclusion assay of earlier passages of the same cell lines. DST of long-term SCLC cultures can predict clinical response and, for etoposide, survival. Disease-oriented panels of carefully selected, continuous, human tumour cell lines can be used to screen new drugs.