Red cell survival was measured in ten subjects with S-C disease and one with S-O Arab (alpha 2 beta 2-121 glu yields lys) disease using both DF32p and 51Cr as tags. Red cell volume was slightly reduced in most patients (87% plus or minus 20% of predicted normal). In nine SC patients, mean red cell life (DF32p) was 28.9 plus or minus 4.0 days. For one SC subject it was significantly longer (47.9 days), as it was for the one with S-O Arab. The S-O Arab subject had irreversibly sickled cells in the peripheral blood, shereas those with SC had few (less than 1/1000 red cells) or none. The S-O Arab hemolysate gelled at a hemmoglobin concentration (16.2 g/100ml) near that for sickle cell anemia hemolysates (15.9 plus or minus 1.0 g/100 ml; n equals 8) but significantly lower than that for SC hemolysates (21.6 plus or minus 1.9 g/100 ml; n equals 5). It seems likely that properties of S-C red cells other than their relative ease of sickling contribute significantly to their rate of hemolysis.