Rat CD4+ T cells are divided phenotypically by the anti-CD45RC monoclonal antibody OX22 into subsets with contrasting functions. Stimulation of T cells in vitro is known to induce a change in isoform from CD45RC+ to CD45RC-. We have investigated the in vifro conditions which promote a switch in isoform in the opposite direction. We observed that a majority of CD45RC-CD4 Tcells (> 90%) spontaneously re-expressed CD45RC during the first 1-3 days of culture in both the presence and absence of alloantigen. The Tcells remained CD45RC+ when cultured for 7 days in serum-free growth medium. However, alloantigenactivated lymphocytes, expressing the interleukin-2 receptor (IL-2R), downregulated CD45RC by day 4 and remained CD45RC-during the course of the experiment. Using mixtures of allotype-marked CD45RC+ and CD45RC-T cells, it was demonstrated that each subset showed comparable survival, IL-2R expression and time courses of activation in response to alloantigen. The repertoire of neither subset was, therefore, deficient in terms of allorecognition. The rapid re-expression of CD45RC in culture was accompanied by a change in function: CD45RC+ "converts", obtained by overnight culture of CD45RC-T cells, induced significantly higher graft-versus-host responses. Thus, the transition in culture from CD45RC-to CD45RC+ reflects a major functional reprogramming of the cell and not a trivial modulation of a surface antigen.