Premature neonates require blood transfusions, and biological parents may wish to be directed donors. Biological mothers pose a potential danger because their plasma may contain antibodies that will react with blood cell antigens inherited by the infant from the father. We studied 25 healthy, pregnant women at the time of delivery for the presence of antibodies against red blood cell, leukocyte and platelet antigens. Mothers known to have red cell antibodies earlier in pregnancy were excluded, and no new red cell antibodies appeared at delivery. Antileukocyte and antiplatelet antibodies were found in 16 and 12% of mothers, respectively. Because these antibodies have the potential to cause adverse reactions when transfused passively, we suggest that either biological mothers not provide blood components containing plasma for their neonates or that maternal red cells and platelets be given as washed products.