Somatomedin A by radioreceptor assay and somatomedin B by radioimmunoassay were measured in serum from women immediately after parturition and from their newborns. The mean levels of somatomedin A in both mothers (0.54 U/ml) and infants (0.50 U/ml) were significantly decreased compared to a reference group consisting of 21 non-pregnant women (0.91 U/ml). There was no difference between the mothers and their children. The mean somatomedin B value in serum from the mothers (63.9 microgram/ml) was above that found in non-pregnant women (19.2 microgram/ml) and the mean value in cord blood and serum from the infants (7.1 microgram/ml) was below it. A positive correlation was noted between somatomedin A and B in cord blood; r = 0.78. The combined somatomedin A and B values in cord blood were positively correlated to birth weight (r = 0.51, P less than 0.05).