Aggregation of red blood cells (RBC) is a major determinant of blood viscosity and of blood circulation through vessels with slow flow (i.e. veins). RBC aggregation and plasma fibrinogen were studied in placental blood samples from 25 neonates with 24 to 41 wk of gestation and in blood from 13 normal adults. The rate and final extent of RBC aggregation were measured by means of a rheoscope (increase in light transmission during blood stasis). Both the rate and extent of RBC aggregation were low in the premature infants, increased with gestational age, and reached the highest values in the adults. Blood from seven infants with 24 to 28 wk of gestation did not show any significant RBC aggregation during the first minute of stasis. RBC aggregation was closely related to the fibrinogen level. Cross-suspension studies (neonatal RBC in adult plasma and adult RBC in neonatal plasma) showed that neonatal and adult RBC had the same aggregation pattern when they were suspended in the same plasma. Moreover, neonatal and adult RBC demonstrated the same strong aggregation when they were resuspended in 1% dextran. These results indicate that specific plasma properties are responsible for the decreased RBC aggregation observed in the neonates while their specific RBC properties do not affect RBC aggregation.