Proliferating cell nuclear antigen (PCNA), also called cyclin, is an intranuclear polypeptide whose synthesis reaches its maximum during the S-phase of the cell cycle. PCNA is expressed in several kinds of proliferating cells, one of which is the mitogen-stimulated human peripheral blood lymphocyte. PCNA expression increases from the late G1 phase through the S-phase of the cell cycle. This study is focused on the regulation of PCNA expression during the G1 phase of human T lymphocytes and on the relationship between PCNA expression and the rate of T cell proliferation. Special use is made of human autoantibodies to PCNA and the development of flow cytometry to label this intranuclear polypeptide. Unstimulated purified human peripheral blood T cells were PCNA negative. T cells treated with monoclonal antibody (64.1) to the CD3 complex expressed receptors for IL 2 but did not express PCNA until exogenous IL 2 was added to the culture. PCNA expression as well as the entry of the cells into S-phase could be inhibited by IL 2 receptor antibodies. Transferrin receptors were expressed only in T cells that were stimulated with both 64.1 and exogenous IL 2. Transferrin receptors were detectable before the cells expressed PCNA. Thus, the onset of PCNA expression is a phase in cell proliferation that follows transferrin receptor expression but preceded DNA synthesis. Drugs like dexamethasone and cyclosporin, which affect the early part of G1, inhibited PCNA expression; whereas cytarabine (ara-C) and hydroxyurea, which affect the S-phase and prevent DNA synthesis, did not block PCNA expression. There was a close correlation between the number of PCNA-positive cells and the number of S-phase cells in unperturbed T cell cultures. The highest level of PCNA expression was seen in cells that were in the first cycle after stimulation. The results show that PCNA expression is regulated by a signal after IL 2 binding and that PCNA labeling is a precise indicator for human T cells that are committed to DNA synthesis. Comparing these results with previous observations on the expression of some other activation-associated antigens, we suggest that the onset of PCNA expression represents a discrete step in T cell activation.