A soluble adenosine triphosphate (ATP)-dependent proteolytic system has been detected in human peripheral blood erythroid cells. Hemolysates prepared from reticulocyte-rich blood of subjects with autoimmune hemolytic anemia, treated pernicious anemia, and iron deficiency anemia or from pools of red blood cells enriched for reticulocytes by density gradient centrifugation were tested against a radioactive casein standard. Up to 57% of the casein was rendered trichloroacetic acid (TCA) soluble after incubation with such hemolysates for 60 minutes in the presence of 1.0 mmol/L ATP. In the absence of ATP or in hemolysates prepared from reticulocyte-poor blood as little as 6% to 10% of the casein was hydrolyzed. The proteolytic activity was found in the 100,000-g supernatant of active hemolysates and was blocked by hemin, N- ethylmaleimide, and sodium vanadate and thus resembles a previously described activity in rabbit reticulocytes. In the presence of ATP, similar lysates prepared from rabbit reticulocytes preferentially hydrolyzed the abnormal human hemoglobins Leiden and Gun Hill compared with hemoglobin A. These results suggest that there is an active ATP- dependent proteolytic system in young human erythroid cells that can degrade certain abnormal globin chains; the enzymatic activity is lost in the transition from reticulocyte to erythrocyte.