Myoglobin levels were measured in 70 maternal and 74 cord sera drawn immediately after delivery. The mean cord myoglobin was 28 ng/ml. Myoglobin levels increased with birthweight, and black infants tended to have slightly higher levels than white babies. Myoglobin levels were not elevated in babies with fetal distress (evidenced by abnormal monitor tracings or meconium staining) compared to babies with no distress. Maternal myoglobin (18 +/- 15.3 ng/ml) was about 10 ng/ml lower than cord levels and there was little correlation between the two. Mothers differed among themselves only with respect to complications of labor and delivery. Those with complications had significantly higher myoglobin levels (29 +/- 24.1 ng/ml) than mothers without difficulties (15 +/- 10.0 ng/ml, P less than 0.05). The fetal myoglobin level was relatively independent of the mother's. While high or potentially toxic myoglobin levels were not observed in this study, it appears that the fetus may be protected by the placental barrier.