Milk protein concentrations were determined either by double antibody radioimmunoassay (IgA and IgG) or single radial immunodiffusion (lactoferrin) in the mammary secretions of seven healthy non-lactating subjects and eight patients with breast disease. IgA and IgG were detected in all samples of breast secretion (whether from normal or diseased breasts) and the concentrations observed were very similar to those in post-partum colostrum and milk. However, because the volume of secretion obtained was very small compared with colostrum and milk, total IgA synthesis by the non-lactating breast is very much less than in the lactating breast. The IgA detected in the mammary secretions was demonstrated to be secretory IgA by gel filtration and it is therefore suggested that the secretory immune system is functional in the non-lactating breast.