The phagocytic activity of peripheral blood mononuclear phagocytes for Staphylococcus epidermis of healthy non-pregnant and pregnant women throughout pregnancy was examined in relation to gestational age. The study included 30 healthy non-pregnant women and 90 healthy pregnant women equally distributed across the three trimesters. Two variables were investigated: the number of mononuclear cells in phagocytosis and the average number of bacteria per monocyte in phagocytosis. As pregnancy progressed a gradual and significant increase was found in the number of monocytes in phagocytosis and a significantly higher proportion of macrophages with six or more bacteria per phagocyte.