Induction of respiratory allergy has not been extensively studied in cattle. The potential for involvement of a type-1 hypersensitivity component in bovine respiratory disease caused by Haemophilus somnus was investigated. Many of the acute, fatal pneumonias that develop in young calves may have a type-1 hypersensitivity component. Vaccination with H. somnus and viral vaccines may contribute to the disease process;Groups of calves were sensitized to Micropolyspora faeni or H. sommus organisms, ovalbumin or equine sera (with and without aluminum hydroxide), or commercial H. sommus bacterins and modified five virus vaccines. This work presents methodology and data to indicate that: (1) Calves can become sensitized to H. somnus after a single exposure and that the presence of aluminum hydroxide may enhance this process. (2) Commercial H. somnus bacterins can induce a significant IgE response that is detectable as early as 14 days after the primary injection. This research also suggests that H. somnus - specific IgE antibodies can be found in serum of some cattle, possibly induced by existing or previous sensitization. (3) Commercial H. somnus bacterins induced production of eicosanoids associated with bronchoconstriction (PGF1alpha and TxB4) and edema formation (6-ketoPGF 2alpha and LTB4) that can be detected as early as 14 days after the primary injection. (4) Modified live Bovine Respiratory Syncytial Virus (BRSV) vaccination can alter these eicosanoid levels and white blood cell populations in calves previously vaccinated with H. somnus bacterins. (5) Haemophilus somnus bacterins and a modified live BRSV vaccine can synergize to enhance higher plasma concentrations of eicosanoids associated with development of a hypersensitized state and a late-phase asthmatic-like bronchoconstriction (LTB4, LTC4, and TxB2). (6) Haemophilus somnus produces histamine and may contribute to development of a type-1 hypersensitivity, and airway hyperresponsiveness in bovine respiratory disease. This work may partially explain the more severe clinical disease occasionally observed in vaccinated cattle compared to nonvaccinated cattle.