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 CDS+, 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+ > CD8+, CD45RA+ > CD4+, CD45R0+ > CD4+, CD45RA+, The phenotype of CDS+ T cells in rejecting kidneys was similar to in vitro alloantigenactivated CDS+, 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.