Zinc has been shown to be involved in many functions of the immune system. This study was conducted to examine the effect of zinc supplementation on phagocytic, fungicidal and metabolic activity of blood monocytes of marasmic infants during nutritional rehabilitation. A controlled, double-blind design was used in which 19 infants fed a zinc-fortified formula were compared with 20 infants fed the same, unfortified formula. Evaluation of phagocytic-fungicidal capacity, growth, zinc, copper and iron status was performed in both groups on admission and after 60 and 105 days of nutritional rehabilitation. Although energy, copper and iron intakes were similar in the two groups, a decrease in the number of infants able to phagocytose one or more Candida buds was observed after 60 days of zinc supplementation compared to admission (p < 0.03). No change in phagocytic ability was detected between admission and 60 days in the control group. The number of infants with depressed fungicidal activity increased significantly after 105 days of nutritional rehabilitation in the zinc-fortified group as compared to controls (p < 0.04). The number and duration of impetigo episodes was significantly greater in the group fed the zinc-fortified formula. These results suggest that zinc supplements at the RDA level may impair monocyte function.