This article reports correlations among gamma-glutamyltransferase (GGT), fetal haemoglobin (fH), alpha-fetoprotein, 5'-nucleotidase, ceruloplasmin, and direct, indirect, and total bilirubin in the serum of blood taken from the umbilical cords of 128 newborns delivered after 37-42 weeks of gestation. GGT was significantly correlated with alpha-fetoprotein, but not with direct bilirubin, indirect bilirubin, total bilirubin, fH, or %fH. Neither fH nor %fH were correlated with alpha-fetoprotein, but there was highly significant negative correlation between both fH and %fH on the one hand, and gestational age and weight at birth on the other. The %fH was also correlated negatively with ceruloplasmin, which in turn exhibited negative correlation with alpha-fetoprotein. The predominant forms of GGT in umbilical cord and adult sera were, respectively, those with alpha 1 and alpha 2 mobility. In cord sera, delipidation with n-butanol brought about loss of GGT activity and a shift from an alpha 1 to an alpha 2 position, whereas no significant effect of this kind was observed in adult sera. Affinity chromatography through Concanavalin A-Sepharose showed cord sera to contain a proportion of bound-GGT (68.5 +/- 5.5%) that was significantly greater (p less than 0.001) than that found in adult sera (59.8 +/- 10.2%). It is concluded that the high GGT activity of cord sera is probably due to hepatic immaturity rather than maternal sources, enzymatic induction or microsomal lesions; that the predominant form of GGT in cord serum may be a complex with HDL and less sialized than the adult enzyme; and that, of the factors examined, the best indicator of neonatal maturity is fetal haemoglobin.