Chronic protein insufficiency in utero was achieved by feeding to rat dams an 8% protein diet beginning before breeding and continuing until weaning. Thereafter, the in utero malnourished pups were fed the 8% protein diet. Superoxide dismutase (SOD), an enzyme which protects against superoxide radical, was quantitated in pellet and supernatant of polymorphonuclear leukocytes (PMN) and alveolar macrophages (AM). The AM of rats malnourished in utero contained 1.7 times as much protein and 3 times as much total SOD activity as the AM of control rats. In PMN of these rats, the amount of protein and SOD activity was the same. Significantly larger number of Streptococcus mutans were located from the dental plaque of the in utero malnourished rats. AM isolated from female guinea pigs fed a 9% protein diet from weaning at 11 days of age contained 1.5 times as much protein and twice as much SOD activity as AM from normal guinea pigs. However their PMN contained smaller amounts of total protein and SOD activity. The malnourished guinea pigs had a mammary gland infection and a prolonged conjunctival infection following inoculation with Guinea Pig Inclusion Conjunctivitis indicating that these animals were more susceptible to these infections. The increase in total SOD activity and in the total amount of protein in AM suggests that chronic protein insufficiency leads to susceptibility to infection and to macrophage activation but has no similar effects on PMN.