Background: In a previous work, we demonstrated with flow cytometry (FCM) methods that accumulation of human cyclin B1 in leukemic cell lines begins during the G 1 phase of the cell cycle (Viallard et al., Exp Cell Res 247: 208-219, 1999). In the present study, FCM was used to compare the localization and the kinetic patterns of cyclin B1 expression in Jurkat leukemia cell line and phytohemagglutinin (PHA)-stimulated normal T lymphocytes. Methods: Cell synchronization was performed in G 1 with sodium n-butyrate, at the G 1 /S transition with thymidine and at mitosis with colchicine. Cells (leukemic cell line Jurkat or PHA-stimulated human T-lymphocytes) were stained for DNA and cyclin B1 and analyzed by FCM. Western blotting was used to confirm certain results. Results: Under asynchronous growing conditions and for both cell populations, cyclin B1 expression was essentially restricted to the G 2 /M transition, reaching its maximal level at mitosis. When the cells were synchronized at the G 1 /S boundary by thymidine or inside the G 1 phase by sodium n-butyrate, Jurkat cells accumulated cyclin B1 in both situations, whereas T lymphocytes expressed cyclin B1 only during the thymidine block. The cyclin B1 fluorescence kinetics of PHA-stimulated T lymphocytes was strictly similar when considering T lymphocytes blocked at the G 1 /S phase transition by thymidine and in exponentially growing conditions. These FCM results were confirmed by Western blotting. The detection of cyclin B1 by Western blot in cells sorted in the G 1 phase of the cell cycle showed that cyclin B1 was present in the G 1 phase in leukemic T cells but not in normal T lymphocytes. Cyclin B1 degradation was effective at mitosis, thus ruling out a defective cyclin B1 proteolysis. Conclusions: We found that the leukemic T cells behaved quite differently from the untransformed T lymphocytes. Our data support the notion that human cyclin B1 is present in the G 1 phase of the cell cycle in leukemic T cells but not in normal T lymphocytes. Therefore, the restriction point from which cyclin B1 can be detected is different in the two models studied. We hypothesize that after passage through a restriction point differing in T lymphocytes and in leukemic cells, the rate of cyclin B1 synthesis becomes constant in the S and G 2 /M phases and independent from the DNA replication cycle.