The study aims to explore ideas about bad milk found among women in Guinea-Bissau. Interviews were held with 20 elderly knowledgeable rural women. Interinformant agreement was high within each ethnic group studied. All the informants recognized colostrum but disliked its consistency. Depending on ethnical background, it was considered good, of no special value or harmful to the newborn baby. Further, all the informants held that mature breast milk could turn bad, e.g. in case of mother's sickness or adultery. Suspected bad milk can be diagnosed by putting an ant into it to observe if it dies. The condition of bad milk can be treated by various procedures. The findings are discussed in relation to similar ideas existing in other societies and to views on the quality of maternal milk held in the industrialized countries. It is proposed that the idea of producing bad milk may be an important determinant of breastfeeding performance generally. Restrictions imposed on the breastfeeding woman, with the intention of producing healthier breast milk, may actually contribute to a decline in breastfeeding.