To determine whether there is any difference between the two epipodophyllotoxin derivatives etoposide and teniposide in their therapeutic effect in small cell carcinoma of the lung (SCCL), they were compared against five human SCCL cell lines in vitro. When the two were compared at equimolar concentrations teniposide was found to be 8-10 times more potent than etoposide, both with 1-h incubation and with continuous incubation in a clonogenic assay and in inducing cell cycle perturbations monitored by flow cytometry. Published pharmacokinetic data suggest that this potency difference is not accompanied by an equivalent increase in toxicity. The concentrations used for the 1-h incubation were about 100-fold the concentrations used in the experiments with continuous incubation to obtain the same degree of cell kill for both drugs. This suggests that they should be given according to a continuous rather than an intermittent schedule.