Stimulated splenocytes were used as a model system to investigate the effects of topoisomerase inhibitors on normal, non-transformed, non-tumoral proliferating cells. The concerted action of camptothecin (a poison of topoisomerase I) and etoposide (a poison of topoisomerase II) lead to nearly complete inhibition of DNA synthesis in concanavalin A-stimulated splenocytes. Analysis of replicated cellular DNA after a short treatment with both drugs revealed a DNA cleavage to medium size fragments. This effect was additive, suggesting that cleavable complexes were formed independently by both topoisomerases on their respective DNA sites. In contrast, prolonged contact with both drugs was followed by degradation of the bulk cellular DNA to nucleosome size fragments, indicating that apoptosis took place in these cells. Combination of camptothecin and etoposide enhanced this phenomenon, consistent with the fact that degradation was the result of secondary events which may amplify the signal. Thus, aphidicolin, an inhibitor of eukaryotic replicases which blocks replication, also triggered DNA degradation in proliferating splenocytes.