Culture fluids and washings of agar cultures (Ex antigens) and phenol-water extractions of whole bacteria or bacterial cell walls were prepared from capsulated and non-capsulated H. influenzae strains. The preparations were subjected to comparative immunodiffusion analyses as well as to studies of their endotoxic effect, as demonstrated by dermal Shwartzman reaction and lethal tests. Immunodiffusion and immunoelectrophoresis analyses showed that in addition to the capsular precipitinogen a second precipitinogen, designated 2, was demonstrated in all preparations of all the strains tested. Both Ex antigens and phenol-water preparations from capsulated strains and from the non-capsulated variants could provoke a dermal Shwartzman reaction in rabbits. Ex antigens from capsulated strains and from a non-capsulated variant were lethal when given parenterally to rabbits. A relation between precipitinogen 2 and the endotoxic effect of the materials was shown. This precipitinogen was demonstrated in washings of young agar cultures as well as in phenol-water extractions of whole bacteria and of cell walls, from both M and S strains. Thus, precipitinogen 2 may be designated an·antigen. It was found to be antigenetically complex, and all the strains tested shared one of the antigen determinant of this complex.