Marked lymphocytosis occurs after exercise. In a study of healthy volunteers this was dominated by one population lacking T cell and B cell determinants and another expressing the Leu 2a phenotype (cytotoxic/suppressor). Lymphocytes from two individuals were characterised further and a near five-fold increase in cells expressing antigens associated with natural killer (NK) cells (Leu 7 and Leu 11) was noted. In addition, these emergent lymphocytes, unlike most T cells, lacked acid alpha-naphthyl esterase activity. In functional studies, exercise led to significantly greater NK activity but, in spite of altering the distribution of lymphocyte subpopulations, there was no detectable change in the proliferative response to the T cell mitogen, concanavalin A, over a wide range of cell concentration, mitogen dose and time. The numbers of low density macrophages and dendritic cells increased concomitantly with the increase in total lymphocytes. We conclude that exercise increases the proportion of circulating NK cells and cells expressing the Leu 2a phenotype.