Glycoproteins of 90 to 95 kD Mr are involved in adhesion to high endothelium and migration into secondary lymphoid tissues in several species. Recent evidence indicates that in primates, one type of these molecules may be subsumed under the CD44 grouping of lymphocyte differentiation Ag. Flow cytometric analysis of circulating macaque lymphocytes for expression of these structures revealed a prominent bimodal distribution, defining two subpopulations with a 5- to 10-fold difference in immunofluorescence staining intensity. Studies of lymphocytes from different anatomic compartments revealed marked differences in the relative numbers of these two phenotypes: splenic and peripheral blood lymphocytes were mostly CD44hi, whereas thoracic duct lymphocytes, which have recently exited lymph nodes and Peyer's patches, were predominantly CD44lo, suggesting that these subsets may have different migratory behavior in vivo. Activated lymphocytes, as defined by light scatter measurements, expression of IL-2-R, and CD45R levels, were all CD44hi. Simultaneous three parameter flow cytometric determination of CD44 and CD45R expression, and cell-cycle analysis with 7-amino actinomycin D further demonstrated that intrinsically cycling cells were contained in the CD44hi, CD45R+ subset. After stimulation of CD44lo lymphocytes in vitro with cross-linked anti-CD3 antibodies and PMA, CD44 expression markedly increased. These data indicate that CD44 up-regulation is an early event in T lymphocyte activation in vivo, and support the hypothesis that patterns of lymphocyte traffic are regulated in concert with cell activation, possibly by differential expression of CD44.