Peripheral blood monocytes isolated from patients with congenital hemolytic anemias, hereditary xerocytosis and spherocytosis, demonstrated in vivo engulfment of red cell and platelet fragments. In addition, morphometric studies performed on these monocytes showed an increase in cytoplasmic/nuclear ratio as well as lysosome and phagosome volumes. The production of carbon dioxide from glucose-1-14C in abnormal monocytes was increased (15–80%) but the intracellular values of beta-glucuronidase and esterase activity were similar to control monocytes. Monocyte locomotion assessed in the presence of chemotactic stimuli was found significantly increased (73 ± 12 monocytes/oil immersion fields vs. 46 ± 5 for control monocytes). We concluded that the monocytes in hemolytic anemias associated with increased in vitro red cell fragmentation have some features resembling the ‘stimulated’ monocytes and that this alteration may be due to red blood cell fragment in