Jurkat T lymphocytes were treated with daunorubicin and WP631, a daunorubicin-based DNA binding agent, in experiments aimed to analyze cellular uptake of these drugs and their effect on cell viability. WP631 was taken up more slowly than daunorubicin, but laser confocal microscopy and spectrofluorometric quantification showed that the drug accumulated in the cells. Despite the slow uptake rate, the antiproliferative capacity of WP631 (measured as IC50 after a 72-h continuous treatment) was greater than that of daunorubicin. The propensities of daunorubicin and WP631 to promote apoptosis were compared. Our results indicate that the major effect of WP631 was a G2/M arrest followed, after about 72 h of treatment, by polyploidy and mitotic (reproductive) death. In contrast, daunorubicin induced a rapid response with classic features of apoptosis.