Non-typeable Haemophilus influenzae (NTHi) is an important opportunistic pathogen of the human respiratory tract that has proved recalcitrant to vaccine development and shows increasing prevalence of antibiotic resistance, prompting us to search for alternative anti-microbial strategies. Haemophilus haemolyticus is closely related to NTHi, and also colonises the respiratory tract, but is recognised as a non-pathogenic commensal species. We identified two H. haemolyticus isolates secreting a 27-kDa protein that inhibited the growth of NTHi in vitro, but did not inhibit a range of other respiratory flora that were tested, suggesting a level of species-specificity against NTHi. A gene knockout established that the gene product of interest was responsible for inhibitory activity. Spectroscopic and x-ray crystallographic analysis of the recombinant protein identified a haem binding site. The protein shares structural features with some iron scavenging proteins, but the haem-binding site is unique. Our data suggest that the protein plays a positive role in haem acquisition in some H. haemolyticus isolates, and that the inhibition of H. influenzae is likely to involve competition for nutrient haem. The work is ongoing in the context that strains of H. haemolyticus might be developed as respiratory probiotics to combat colonisation and infection with NTHi.