Flavopiridol is a novel semisynthetic flavone derivative of the alkaloid rohitukine. Flavopiridol is known to inhibit potently the activity of multiple cyclin-dependent kinases. We have assessed its effects on normal and malignant cells in preclinical animal models of localized and disseminated human hematopoietic neoplasms. Flavopiridol, when administered as daily bolus intravenous (IV) injections, produced selective apoptosis of cells in the thymus, spleen, and lymph nodes, resulting in atrophy of these organs. With the exception of the intestinal crypts, apoptosis or tissue damage was absent in all other organs investigated (kidneys, liver, lungs, bone/bone marrow, muscle, and heart). Flavopiridol had a marked apoptotic effect documented by DNA nick-end labeling, or DNA agarose gels in xenografts of human hematopoietic tumors HL-60, SUDHL-4, and Nalm/6. After treatment with 7.5 mg/kg flavopiridol bolus IV or intraperitoneal on each of 5 consecutive days, 11 out of 12 advanced stage subcutaneous (s.c.) human HL-60 xenografts underwent complete regressions, and animals remained disease-free several months after one course of flavopiridol treatment. SUDHL-4 s.c. lymphomas treated with flavopiridol at 7.5 mg/kg bolus IV for 5 days underwent either major (two out of eight mice) or complete (four out of eight mice) regression, with two animals remaining disease-free for more than 60 days. The overall growth delay was 73.2%. The acquired immunodeficiency syndrome-associated lymphoma AS283 showed no significant response when flavopiridol was used in advanced s.c. tumors, but when treatment was initiated in early stages, there was a complete regression of the early tumors, and a significant overall growth delay (>84%). When flavopiridol was used in severe combined immunodeficient mice bearing disseminated human acute lymphoblastic leukemia Nalm/6 cells, there was 15-day prolongation in survival (P = .0089). We conclude that flavopiridol greatly influences apoptosis in both normal and malignant hematopoietic tissues. This activity was manifested in our study as a potent antileukemia or antilymphoma effect in human tumor xenografts, which was dose and schedule dependent. These findings provide compelling evidence for the use of flavopiridol in human hematologic malignancies.