PIP: Prolonged breastfeeding has been reported to protect the child from grossly defective weaning foods used in most developing countries. Even grossly malnourished mothers have been known to produced enough milk to keep their children alive. Hytten and Thompson reported that there is no evidence that maternal nutrition affect lactation. This study investigates the lactation performance of an unselected group of nursing mothers from a poor to moderate socioeconomic background, and determines the possible effects of their nutritional state on their breast milk and their infants. 41 urban mothers of a moderate to poor socioeconomic status (aged 18 to 38 years) and with an average of 4 pregnancies resulting in 3 surviving children comprised the study population. The mothers were divided into malnourished M group (n=17) and clinically apparently healthy H group (n=24). Biochemical assessments were made of the mothers and their infants, and an NSIM (Nutritional State Index of the Mother) and NSII (Nutritional State Index of the Infant) were made. The malnourished state of the mothers of group M was confirmed by dietary histories and biochemical assessments. 9 mothers in the H group had 'low' serum albumin values, and only 5 had a urea nitrogen/creatinine nitrogen ratio of above 30. The data suggest that the women were consuming a diet poor in protein but adequate in calories. Both groups of mothers produced milk of suboptimal but nonetheless acceptable composition, those in group M having lower concentration of protein and calories than those in group H. On the average, group H mothers produced a normal amount of milk, while group M mothers produced 22% less milk. Most of the infants were found to be of suboptimal nutritional status like the mothers. There was a high correlation between the NSII and amount of milk and of proximate constituents. NSIM correlated very highly with NSII. Nutrition of pregnant women should be improved to control infant malnutrition.