Fifteen three- to six-month old Hereford-cross calves were divided into three groups. The first group was inoculated with bovid herpersvirus 1 (Strain 108), the second with a commercial intranasal vaccine against bovid herpesvirus 1 and the third group acted as controls. At least three weeks after vaccination, all calves were weaned, placed in an environmental chamber at 25.0 degrees C (days) and -13.3 degrees C (nights) and challenged with an aerosol of bovid herpesvirus 1 followed four days later by an aerosol of Pasteurella haemolytica. All surviving calves were sacrificed four days after the second aerosol. None of the calves inoculated with bovid herpesvirus 1 virus or the commercial vaccine developed a generalized pneumonia, although there were one or two nodules (4--8 mm diameter) in two of the calves given the commercial vaccine. Four of the five control calves had extensive lobar pneumonia at necropsy, two of the five died from the disease. Details of the clinical, pathological, bacteriological, virological and some of the serological findings are reported.