Ribonuclease (RNAse) activity was measured in placentas from a sample of 49 urban Guatemalan women from low (30) and high (19) socioeconomic status (SES). Low SES women had a larger proportion (40%) of high levels of RNAse (> 60, 000 units/g) than women from the high SES group (5 %). A trend to inverse association (p = 0.06) was also observed between amount of supplemented food energy during pregnancy and RNAse activity in rural populations covered by two food supplementation programs (protein energy and energy). Two indicators of maternal nutrition, third trimester weight and height, also showed inverse associations with placental RNAse activity. It is inferred from these results that improved maternal nutrition decreases placental RNAse activity. Moreover, high levels of placental RNAse activity were associated, up to 36 months of age, with higher proportions of: physical growth retardation in weight, height and head circumference; below-average psychological test performance; and infant deaths. These results deserve consideration from both the physiological and public health points of view. © 1977, Gordon and Breach Science Publishers Ltd.