Changes in CD45R expression correlate with changes in phenotype in mouse, rat and human T cells. It has been shown in mouse that CD45RB high T cells produce mostly interleukin-2 (IL-2) while CD45RB low T cells produce more IL-4 than IL-2 after mitogen stimulation in vitro. CD45RB expression also decreases when T cells are stimulated. In this study we compared responses of CD45RB high and low CD4+ T cells to alloantigens. Although a majority of unstimulated murine T cells from unimmunized mice are CD45RB high, we were able to isolate and purify sufficient numbers of T cells to study their response to alloantigens. When separated cells were stimulated in vitro with alloantigen the CD45RB high T cells became heterogeneous for their expression of CD45RB, indicating that high cells decrease their expression of CD45RB epitopes. Surprisingly, only CD45RB high T cells from unimmunized mice were alloreactive, as measured by proliferation and lymphokine secretion. In contrast, both CD45RB high and low populations from mice primed with allogeneic spleen were responsive to alloantigens. The 'primary' response of T cells from alloantigen-primed mice to third party stimulators is 10-fold greater in the CD45RB high population than in the CD45RB low, as would be predicted by our results in the primary mixed lymphocyte/leucocyte reaction (MLR). Furthermore, the response of CD45RB low T cells from unprimed mice and the response to third party alloantigen from primed mice was reconstituted by the addition of exogenous IL-2. The response of CD45RB low cells from primed mice was specific, as the third party alloresponsive cells were again primarily contained within the CD45RB high population. In the CD45RB high population in the secondary MLR we observed an increase in the production of IL-4 relative to IL-2.