A group of mAb recognizing the 200- and/or 220-kDa determinants (CD45R) of the leukocyte common Ag such as 2H4, WR16, MD4.3, and SN130 cross-block each other showing that they recognize a closely related epitope. The antibody UCHL1 reacts with a 180-kDa determinant of the leukocyte common Ag and exhibits a reciprocal T subset distribution pattern to the CD45R group. Peripheral blood T cells were 40% positive for UCHL1 and 58 to 65% positive for the CD45R antibodies; less than 1% of cells stained for both. On activation of CD45R+,UCHL1- T cells by PHA, up to 40% of cells became positive for both CD45R and UCHL1 by day 3. By day 7, CD45R+,UCHL1- cells fell from 90 to less than 21% whereas UCHL1+,CD45R- cells rose from 2 to 93%. Conversely, PHA-stimulated UCHL1+,CD45R- cells remained UCHL1+,CD45R- during the 7 days in culture showing that phenotypic change was unidirectional from CD45R+ to UCHL1+. In primary allogeneic mixed lymphocyte reactions, activated CD45R+ T cells also showed a change to UCHL1+. When these cells were rechallenged by the original alloantigen, the UCHL1+ cells showed 7- to 20-fold greater proliferation than the CD45R+ cells on day 3 after rechallenge. The recovery of virtually all alloantigen induced secondary proliferative response in the UCHL1+,CD45R- T cell population suggests that UCHL1 identifies a primed population of T cells which may include memory cells.