The nuclear protein p53 has been reported to be associated with cell transformation and/or proliferation so that the study of p53 expression in human malignancy has potentially important clinical implications. We have analyzed the p53 expression in mitogen-stimulated and nonstimulated human lymphocytes, in several human leukemia cell lines (Molt-4, Raji, Daudi, HL-60, KG-1, K562 and U937) and in fresh bone marrow (BM) cells. Simultaneous differential staining of p53 (identified by a FITC-labeled monoclonal antibody) versus DNA (stained with propidium iodide, PI), followed by bivariate analysis with flow cytometry (FCM) made it possible to evaluate p53 expression with respect to cell position during the cell cycle. The data show that in stimulated lymphocytes p53 is progressively accumulated during the G1, S and G2-phases, while in non-stimulated conditions most cells are remaining in G0/G1 and express p53 to a lesser degree. This suggests that expression of p53 is more correlated with cell growth than with entrance into (or progression through particular phases of) the cell cycle. Cells from acute lymphoblastic leukemia (ALL) and Burkitt's lymphoma cell lines express elevated levels of p53, while all examined human acute myeloid leukemia cell lines synthesize negligible p53 protein. Understanding the variations in p53 expression in different types of human leukemia may provide some insight into the biologic roles of p53 in normal and malignant cells.