Complexes containing proteins, capsular polysaccharide (polyribosylribitolphosphate [PRP]), and lipopolysaccharide (LPS) had been isolated from the supernatants of cultures of Haemophilus influenzae type b and were found to elicit rabbit antibodies to PRP much more effectively than did purified PRP vaccine. Similar complexes have now been prepared, in which the LPS content and endotoxic activity are greatly reduced. One such preparation, called polysaccharide-protein complex (PC), elicited antibodies to PRP in a "boostable" pattern in weanling rabbits and antibodies to the somatic components of H. influenzae type b, primarily LPS. PC greatly exceeded PRP in the in vitro stimulation of [3H]thymidine incorporation by mononuclear leukocytes from human peripheral blood; the response occurred in T cells rather than in B cells. Lymphocyte stimulation by conventionally purified PRP appeared to be due to a small residuum of material that resembled PC.