The effects of season and variations in the prevalence of infectious disease on the concentrations and daily production of breast-milk immunoproteins were studied in 152 rural Gambian mothers and their children up to 26 months post-partum. IgA, IgG, IgM, C3, C4, lactoferrin, lysozyme and secretory component concentrations and breast-milk volumes were measured longitudinally over a six month period which encompassed dry and rainy seasons. No increase in the production of any immunoprotein was observed at the time of maximum prevalence of serious infectious diseases, especially diarrhoea, in the children. Enhanced secretion of certain immunoproteins was noted in mothers of children aged 9-18 months at the beginning of the rainy season. There was some evidence that this may have been associated with skin sepsis, particularly impetigo, in the children. The production of most immunoproteins fell during the rainy season. This was not the result of declining maternal food intakes as similar decreases were seen for women receiving a dietary supplement.