The Brazelton Neonatal Behavioral Assessment Scale was administered to 24 each Puerto Rican, Black, and Caucasian, 2-day-old, full-term healthy neonates. Comparisons on 8 Brazelton scale summary scores showed that Puerto Rican infants scored lower on habituation, higher on orientation, and higher on maintaining their organization with increasing stimulation than Black and Caucasian infants. Multiple regression analysis showed that the combination of biomedical variables was significantly correlated with 6 of the Brazelton scale summary scores for the Puerto Rican infants, 1 summary score for the Caucasian infants, and none for the Black infants; it revealed population differences with respect to the synergistic effects of biomedical variables on neonatal behavior.