SummaryThree antigenic preparations were obtained from a non‐capsulated strain ofHaemophilus influenzaeby ultrasonic disintegration, hot phenol extraction and from a fluid culture. They were designatedH. influenzaecytoplasmic antigen (H(1–5));H. influenzaecell wall antigen (HCW); andH. influenzaeculture filtrate antigen (HCF).Studies showed that H(1–5) antigen contained heat stable and heat labile components. The heat stable fraction stained positively for polysaccharide, had a positive limulus lysate test and there was immunological cross‐reactivity between this and heat stable fractions of HCW and HCF. Limulus lysate assay indicated the presence of endotoxin in HCW and HCF preparations.Heat stable as well as heat labile antigens ofH. influenzaeshould be given consideration in future studies regarding the pathogenicity of this organism in the lower respiratory tree. The specificity of the heat stable antigen ofH. influenzaeneeds to be det