The capsular polysaccharide of Hemophilus influenzae type b, polyribosyl ribitol phosphate (PRP), is released from growing organisms during human infection and can be found in body fluids. It binds to untreated erythrocytes. Many patients with invasive infections with this organism develop significant hemolysis, but the mechanism has been unclear. We have found that PRP binds to human erythrocytes in vivo. PRP-coated erythrocytes have a shortened circulation time in mice, but do not lyse spontaneously or fix complement. PRP-coated erythrocytes exposed to antiserum to H. influenzae type b are undamaged in the absence of complement, but are rapidly and effectively lysed in the presence of an intact complement system both in vitro and in vivo in mice. PRP-coated red cells are taken up by liver and spleen. Antiserum to PRP increases hepatic uptake of PRP-coated red cells more than splenic, and appears to induce intravascular, complement-mediated hemolysis, as well as extravascular hemolysis. Patients with invasive infection develop hemolysis when circulating PRP and antibody to PRP are present simultaneously. PRP can sometimes be detected on patient erythrocytes when free PRP is present in serum, but this is an inconsistent finding. The hemolytic anemia that occurs during human infection with H. influenzae type b may be due to absorption of PRP to red cells and immune destruction of sensitized erythrocytes. The process requires an intact complement system; both complement-mediated cell lysis and extravascular hemolysis contribute to red cell destruction.