In normal individuals, 80 +/- 5% of circulating CD8+ T cells express CD45RA, and 20 +/- 7% of these cells express CD45R0 antigens. After activation, CD8+ cells expressing CD45RA decrease to 56-67% while those expressing CD45R0 increase to 38-67%. Although precursors of alloantigen-specific cytotoxic T cells were found in both CD8+, CD45RA+ and CD8+, CD45R0+ subsets, the specific effector cells were exclusively CD8+, CD45R0. Allospecific cytotoxic CD8+ clones were also entirely CD45R0+. A lectin-dependent cytotoxic (LDC) assay unmasked a hierarchy of killing after alloactivation which was CD8+, CD45R0+ greater than CD8+, CD45RA+ greater than CD4+, CD45R0+ greater than CD4+, CD45RA+. The phenotype of CD8+ T cells in rejecting kidneys was similar to in vitro alloantigen-activated CD8+, CD45R0+ cells and cytotoxic CD8+ clones. Firstly there was an increase in the relative proportions of CD45R0+ (60 +/- 8%) and a decrease in CD45RA+ (35 +/- 10%) CD8+ cells relative to circulating CD8+ subsets. In the rejecting grafts, 34 +/- 9% of the CD8+ cells were also DR+ indicative of recent activation. Furthermore, 16% (range 4-35%) of rejecting CD8+ cells were Ki67+ suggesting that these cells were proliferating. Finally, 17% (range 4-53%) of T cells in rejecting kidneys simultaneously expressed both CD45RA and CD45R0 markers. These results show that in vitro alloantigen-activated CD8+, CD45R0+ cells represent a primed/memory cytotoxic population. In addition, they provide indirect evidence that a proportion of CD8+ cells in rejecting kidneys were actively switching from a naive to a memory phenotype in vivo in a manner analogous to that in vitro.