Cord blood platelet aggregation and thromboxane B2 (TxB2) synthesis were investigated in 20 healthy infants of drug-free healthy mothers with an uncomplicated pregnancy ending at term in a normal vaginal delivery. Eight of the mothers were habitual smokers and the remaining 12 were non-smokers. The platelet count was slightly lower in infants of smoking mothers than in the control infants. There were no differences between the two groups in the qualitative or quantitative aggregation responses (maximal change in optical density) to stimulation by 0.55, 1.1, 3.3 and 6.6 microM adenosine diphosphate, 0.5 and 1.0 mM arachidonic acid (AA) or collagen 1.0 and 5.5 mg/l. Nor was there any difference between the groups in regard to TxB2 synthesis after stimulation by these substances. The amount of TxB2 synthesized merely reflected the occurrence of reversible or irreversible platelet aggregation. Stimulation of platelets by AA induced by far the highest synthesis of TxB2. The generally decreased aggregability of platelets of newborn infants, compared with that in adults, was confirmed in this study. No effect of maternal smoking during pregnancy on the prostaglandin-dependent pathway of aggregation of cord blood platelets was demonstrated.