Effect of tumor growth on the survival of circulating erythrocytes was studied in mice bearing a wide spectrum of experimental tumors. RBC half-life (t1/2), measured by 51Cr-labeling technique, decreased significantly (p less than 0.05) in all the tumor types studied, particularly in transplantable Sarcoma-180 and benzo(a)pyrene-induced primary fibrosarcoma. Changes in erythrocyte morphology like anisopoikilocytosis were also observed in the tumor hosts. Cross-transfusion of 51Cr-labeled RBCs between normal and tumor-bearing animals revealed that both intrinsic and extrinsic factors are responsible for shortened RBC survival. As far as the cellular abnormalities are concerned, the decrease in RBC t1/2 was not attributable to increased osmotic fragility as the cells were observed to be osmotically more resistant. Similarly, membrane sialic acid content was markedly elevated in the tumor hosts, thus the shortened erythrocyte life-span cannot be attributed to decrease in sialic acid content of the erythrocyte membrane.