The antibody content of milk from healthy Swedish mothers was compared with that of milk from mothers of a very low socio-economic group in a developing country. Antibodies of various immunoglobulin classes against E. coli O antigens were determined with the enzyme-linked immunosorbent assay (ELISA). The milk antibodies which mainly belonged to the secretory IgA class appeared in similar concentrations in milk from the two groups using E. coli antigens of Swedish as well as Pakistani origin. The secretory IgA antibodies could be demonstrated in the stool of the breast-fed infants of the undernourished mothers. Also the concentration of serum IgG and IgA antibodies to E. coli O antigens were similar in the Pakistani and Swedish mothers. The serum IgM antibody levels of the Pakistani mothers were higher, however, presumably due to a higher frequency of infections. It was noted that the milk production decreased considerably upon the hospitalization of the healthy and well-to-do Swedish mothers. The small milk volumes of the undernourished Pakistani mothers suggest that the lactation failure observed was mainly due to inadequate milk flow and not to decreased milk quality. The results indicate the necessity of studying the nutritional, psychological and social factors responsible for low milk yield and add yet another reason to stimulate prolonged breastfeeding.