Six capsulated Haemophilus influenzae strains, types a to f (Ma–Mf), and two non-iridescent, non-capsulated variants of the Ma and Md strains (Sa and Sd) were studied by means of double immunodiffusion and immuno-electrophoretic methods. A capsular antigen, designated precipitinogen 1, and a second antigen, designated precipitinogen 2, were demonstrated in extracellular (Ex) antigen preparations of the Ma–Mf strains. In addition to the serotvpe capsular antigen, traces of capsular antigen of at least one other serotype could be demonstrated in all Ex antigen preparations except in MaEx. In strain Sa, small amounts of capsular antigen a were found, whereas no capsular antigen d was revealed in the strain Sd. The precipitinogens 2 of the Ma and Md strains were also demonstrated in the S variants, indicating that this antigen may not be of capsular origin. Cross-reactions of precipitinogen 2 were seen between strains of three different serotypes (a, e and f).