The percentage of erythrocytes with pits in the peripheral blood after splenectomy was followed in 4 patients. The pit counts started to increase soon after surgery and were above 20% on the 20th postsplenectomy day. The results demonstrate that the majority of pits developed in circulating erythrocytes. Two other patients with autoimmune hemolytic anemia who had failed to respond to splenectomy presented an increase of pit counts from 5.5–13% to 29.8–47.4%, accompanying an increase of erythrocyte life-span during corticosteroid treatment. The data indicate that pits are formed preferentially in older erythrocytes.