Mastitis was found to be a sizeable clinical problem in a group of lactating Gambian mothers. The mean monthly incidence was 2.6% and repeated episodes of mastitis were common. The role of milk antimicrobial factors in the local defence of the breast against mastitis was investigated by analysis of IgA, IgG, IgM, C3, C4, lactoferrin and lysozyme in the breast milk of 10 mastitis patients. Acute inflammation of the breast was accompanied by the rapid appearance of high concentrations of serum-derived immunoproteins in mastitic milk. Changes in the milk levels of lactose, sodium and transferrin indicated that this was due to a temporary opening of the paracellular pathway. Concentrations of secretory immunoproteins (IgA, lactoferrin and lysozyme) exhibited a delayed response, being elevated one week after the attack of mastitis. The normal milk of mastitis sufferers was significantly deficient in IgA, C3 and lactoferrin when compared with other lactating women suggesting that the former were predisposed to mastitis.