Plasma was taken from infants and children with acute hemolytic uremic syndrome (HUS) or following remission from this disease. The capacity of these plasma to stimulate release of prostacyclin-like activity from "exhausted" rat aorta rings was studied. Plasma from 10 out 12 children in the acute phase of HUS, but from only 3 out of 15 children following remission failed to stimulate prostacyclin release (p less than 0.005). These findings suggest that, for the majority of children, the plasma abnormality in the acute phase of the disease is acquired rather than congenital.