Approximately 25% of T cells in young mice are able to extrude the fluorescent P glycoprotein substrate Rhodamine-123 (R123), and these R123low cells are present within the naive and memory cell populations of both CD4 and CD8 type. The proportion of R123low T cells increases with age to approximately 60% in mice older than 18 mo of age. CD4 memory T cells from young mice secrete more IL-4 compared with cells from old donors when activated by anti-CD3 and further cultured for 7 to 10 days in the presence of IL-2. To determine whether this age-related decline in IL-4 production was related to the parallel accumulation of R123low T cells within the CD4 memory subset, we compared IL-4 production in cell preparations enriched for R123high and R123low cells by electronic cell sorting. IL-2-driven IL-4 production by CD3-activated CD4 memory cells was found to be limited almost entirely to the R123high subset, i.e., the subset that declines with age. Proliferation under these culture conditions also declined with age and was also much more vigorous in R123high than in R123low cells. Not all R123low T cells, however, were resistant to IL-2-dependent proliferation and differentiation: limit dilution analyses showed nearly equal proportions of Con A-responsive proliferative and cytotoxic clones within the R123high and R123low subsets of CD8 naive cells. These data show that P glycoprotein-mediated extrusion of R123 delineates CD4 memory T cell subsets that differ in function.(ABSTRACT TRUNCATED AT 250 WORDS)