The exposure of newborn infants to the plasticizer di-(2-ethylhexyl) phthalate (DEHP) and its primary metabolite mono-(2-ethylhexyl) phthalate (MEHP) was studied during exchange transfusions by measuring their contents in the infused blood. Plasma concentrations of DEHP and MEHP in the blood withdrawn from the infants during the transfusions also were determined. The amounts of DEHP and MEHP inadvertently infused varied from 1.7 to 4.2 and 0.2 to 0.7 mg per kg body weight, respectively. Immediately after the transfusions, the plasma levels of DEHP in the individual infants varied between 3.4 and 11.1 micrograms per ml. MEHP in the corresponding samples ranged from 2.4 to 15.1 micrograms per ml. Judging from plasma concentrations of DEHP and MEHP during and after transfusion, there was no gradual accumulation of these substances in the plasma during the course of the transfusion. In the two infants who underwent a second exchange transfusion, significant levels of phthalates were found at 16 and 23 hours, respectively, after the first transfusion. Plasma concentrations of DEHP in these infants declined at a faster rate than those of MEHP, thus pointing to the importance of examining the pharmacokinetics of this potentially toxic metabolite.