While studying the ecology of fetal malnutrition, levels of cord serum IgM were determined by radial immunodiffusion in cord blood taken from seven groups of newborns. Group 1 was selected among the high socioeconomic Peruvian urban population. The other six groups were from the low socioeconomic strata of Peru and Guatemala (Ladino and Indian, urban and rural), among whom, very poor environmental conditions prevailed. The results were compared with values corresponding to the low income black population of Birmingham, Alabama. All IgM levels above 0.19 mg/ml were considered high, since the point of inflexion appeared between 0.19 and 0.20 mg/ml in a log normal probability chart. In Group I, only 6 percent of infants had high IgM values, a value similar to the 4 percent in the Birmingham low-income population. On the other hand, 40 to 65 percent of infants from Groups 2 to 7 showed high IgM levels. It is suggested that one of the main factors responsible for the high IgM concentrations was intrauterine infection. The significance of the findings should be explored, in view of the causal relationship between intrauterine infection and impaired physical and mental development. © 1974, Taylor & Francis Group, LLC. All rights reserved.