In 45 histologically verified breast cancer patients the cell-mediated immune responses were tested prior to surgical therapy. The cell immunity was assessed by the percentage of T and active T-lymphocytes and by measuring the lymphocyte reactivity to phytohemagglutin (PHA) and concanavalin A (Con A). Disease-free interval was longer and the mortality after 3, 5 and 8-year follow-up significantly lower in the group of patients with the normal number of T and active T-lymphocytes than in the group of patients with the lower number of those cells. The proliferative response of lymphocytes to Con A and PHA assessed in allogeneic and autologous serum also seemed to be a useful prognostic parameter. The survival was significantly longer in the group of patients with a normal lymphocyte reactivity to those mitogens than in the non-reactive group.