The tyrosine phosphatase CD45 is alternatively spliced to generate isoforms of different molecular weights (180-220 kDa) which are differentially expressed on hematopoietic cells. Monoclonal antibodies reacting with either the 180-kDa (UCHL-1, CD45RO) or the 200- to 220-kDa (2H4, CD45RA) isoform have been used to subdivide T cell populations based on their expression of one or the other of these two epitopes. CD45RA T cells have "naive" characteristics of unresponsiveness to recall antigens and prominence in cord blood, while CD45RO T cells are considered "memory" T cells because they proliferate to recall antigens and increase following PHA activation of cord blood. However, we have recently demonstrated the expression of the CD45RA isoform on a subpopulation of CD45RO+ T cell clones, suggesting that CD45RA is not a universal marker for naive T cells. Using propidium iodide staining of the DNA to determine cell cycle stage, we now show that CD45RA expression is significantly higher on T cell clones during the S, G2, and M stages of cell cycle when compared to CD45RA expression on cells in Go and G1. Furthermore, CD45RA expression on cells undergoing mitosis is not limited to long-term activated T cell clones, as uncultured peripheral blood T cells in the S/G2/M phase express significantly more CD45RA. The percentage of T cells coexpressing CD45RA and CD45RO also increases following PHA activation, indicating that T cells in the process of division express both isoforms. These results suggest a potential role of the CD45RA isoform during the stages of cell cycle leading to mitosis.