Prolactin (PRL) plays a modulatory role in immune function. Previous studies have demonstrated that physical activity (swimming until exhaustion) provokes a stimulation in the phagocytic function of peritoneal macrophages. In this study, we have investigated the possible role of PRL as a mediator of physical activity-induced stimulation of macrophage phagocytosis. Peritoneal macrophages from BALB/c mice were incubated for 30 min with 1.1 (basal concentration in these animals), 2.2 (the concentration observed in plasma after swimming until exhaustion) and 8, 16 and 22,000 ng/ml of PRL. The results indicated that incubation of peritoneal macrophages with a concentration of PRL similar to that observed in plasma immediately after physical activity stress stimulates phagocytic capacity. This stimulation was also observed after incubation of macrophages with the higher concentrations of PRL. We conclude that PRL may be considered as a mediator of physical activity-induced stimulation of macrophage phagocytosis, confirming the immunoregulatory role of this hormone.