Haemophilus influenzae type b isolates have been subdivided based on differences in major outer membrane protein (OMP) profiles resolved on gradient and modified Laemmli sodium dodecyl sulfate-polyacrylamide gel electrophoresis systems. Although 21 subtypes have been observed, 86% of invasive isolates have one of five common subtypes and 71% of isolates have one of three subtypes. In isolates with two of the most common outer membrane subtypes, one major OMP has an apparent molecular weight of 37,000. In isolates with another common OMP subtype, a cross-reactive protein with an apparent molecular weight of 36,500 is observed. All three proteins have been designated P2. Protein P2 from these prototype isolates was solubilized with Zwittergent 3-14 and purified to homogeneity. Based on amino acid compositions, cyanogen bromide cleavage products, staphylococcal V8 protease, and chymotryptic peptide maps, the P2 protein from the three isolates has been highly conserved. Rabbit antibody prepared against P2 from one strain was cross-reactive with P2 isolated from the other two heterologous strains by Western blot. This antibody passively protected infant rats against type b Haemophilus infection caused by the homologous organism, but not against challenge by a strain with the heterologous 36,500 mol wt P2 protein. Thus, although the P2 protein from isolates with different OMP subtypes are closely related, the protection experiments suggest that determinants on the cell surface interacting with protective antibody may be strain or subtype specific.