Immunoglobulin M (IgM) levels were determined on 129 cord sera; 54 cord sera were obtained from infants delivered between the twenty-second and thirty-seventh weeks of gestation, from 54 term deliveries, and from 21 low-birth-weight, multiple-birth infants. The data show that cord IgM levels are influenced by gestational age and that in evaluating the significance of a given IgM level in a premature-by-dates, low-birth-weight infant this factor must be taken into consideration.