Antithrombin III (AT-III) heparin cofactor activity and its antigen levels have been determined in 106 plasma samples from 42 term and preterm neonates. In contrast to healthy adult controls, a reduced activity/antigen (act/ag) ratio (ranging from 0.26 to 0.86) was observed in 90% of the samples and was independent of the state of health of the infant. By modifying the routine assay techniques, laboratory artefacts were excluded as the cause of the observed discrepancy. The relative increase in antigenic AT-III could not be accounted for by circulating AT-III-thrombin complexes, or by increased heparin cofactor II plasma levels in neonates.