Expression of the human mucosal lymphocyte antigen, HML-1 (CD103), recently identified as a novel alpha E beta 7 integrin, was studied on peripheral blood lymphocytes activated with mitogen or specific antigen. HML-1 was up-regulated on PHA activated T-lymphoblasts cultured in 100IU/ml interleukin-2 (IL-2), reaching a peak of > 50% positive cells at day 7, and expression was maintained at this level throughout the 28-day culture period. Following a transient decrease in the percentage of L-selectin cells, expression of this molecule was maintained on most PHA T-lymphoblasts. Cells activated by purified protein derivative of M. tuberculosis (PPD) or in mixed lymphocyte culture also up-regulated and maintained HML-1 expression for 14 days. In contrast, in all cases the percentage of CD25+ cells rose initially but subsequently declined over the same time periods. When freshly isolated cells from tonsil, spleen, mesenteric lymph node and lung were analysed, only lung contained significant numbers (39 +/- 6%) of HML-1+ cells. In both freshly isolated and activated cell populations the great majority of HML-1+ cells co-expressed CD8 although some HML-1+ CD8- cells were also present. Production of TGF-beta 1 peaked early during T-lymphoblast and MLR cultures and was not related to induction of HML-1 expression. Immunoprecipitation studies showed that the HML-1 molecule expressed on 10-day PHA T-lymphoblasts was indistinguishable from that found on intestinal intraepithelial lymphocytes and that no alpha 4 beta 7 integrin was expressed by these cells. Although HML-1 expression is essentially restricted to mucosal leucocytes in vivo, these experiments show that it is readily induced and maintained along with co-expression of L-selectin following CD8+ T-lymphocyte activation in vitro.