Antithrombin III, a major inhibitor of activated coagulation factors has low immunologic levels in the human infant. The objective of this study was to determine if the antithrombin III molecule is fully functional in sick premature infants. The populations studied included: adult controls (n = 20), full term healthy infants (n = 18), sick premature infants on day 1 (n = 16) and at greater than 7 days of age (n = 10), and infants with disseminated intravascular coagulation (n = 11). This was diagnosed in the presence of prolonged screening tests, decreased levels of fibrinogen, and platelets along with elevated fibrin degradation products. Plasma antithrombin III levels were measured biologically (chromogenic substrate S2238) and immunologically (radialimmunodiffusion), and expressed as a percent of adult pooled plasma. Crossed immunoelectrophoresis were performed in the presence and absence of heparin. The antithrombin III biologic/immunologic ratios for adults, healthy full term infants, and sick premature infants on day 1 of life were all near unity. In contrast sick premature infants beyond the 1st wk of life and infants with disseminated intravascular coagulation had lower biologic activity compared to immunologic (B/I = 0.77 +/- 0.28, 0.78 +/- 0.17, p less than 0.01), respectively. In all groups, the antithrombin III molecule was normal on crossed immunoelectrophoresis except for one infant with disseminated intravascular coagulation. Sick premature infants may acquire a dysfunctional antithrombin III molecule in the postnatal period.