The rapid loss of alloreactivity within populations of antigen‐primed, in vitro propagated T cells cannot he explained by the appearance of suppressor cells nor by the dilution effect of the proliferative antigen‐specific T cells alone. The involvement of trans‐stimulation in loss of alloreactivity, i.e. the recruitment of non‐antigen‐specific T cells into proliferation in an antigen‐dependent and specific fashion, was assessed. Susceptibility to trans‐stimulation was found to correlate directly with stale of activation at the outset of assay. Large T cells (low buoyant density) hut not small T cells (high buoyant density) are susceptible lo trans‐stimulation. Moreover, in vitro pre‐activation of small T cells by mitogen confers susceptibility to trans‐stimulation. Analysis of lire alloreactivity in Percoll fractions of antigen‐primed lymph node T cells revealed activity in both large‐ and small‐T‐cell fractions with some enrichment in the latter. The small T cells, refractive to trans‐stimulation, are diluted out of the population within the early weeks of antigen‐mediated in vitro propagation, accounting for a rapid loss of considerable alloreactivity. The loss of all delectable alloreactivity within antigen‐selected populations suggests that the slate of activation conferring sensitivity to trans‐stimulation must be maintained, and that neither the antigen nor the culture condi