This thesis investigated the statistical association of titres to ' Pasteurella hemolytica, Hemophilus somnus', bovine viral diarrhoea virus and bovine corona virus, with undifferentiated bovine respiratory disease (UBRD) at three feedlots in Ontario, Canada. The prevalence of exposure to the agents prior to arrival at the feedlot and the incidence of infection during the study period were estimated using the "proxy" variables, arrival titre and change in titre. Titres to 'Pasteurella hemolytica ' and bovine viral diarrhoea virus were examined to elucidate their behaviour. However as more is known about the sero-epidemiology of these two organisms they also represented a point of reference for the behaviour of the 'Hemophilus somnus' and bovine corona virus titres. A factorial design was used to randomise vaccination against both 'Hemophilus somnus ' and 'Pasteurella hemolytica'. The nonvaccinated (for each antigen) animals served as monitors of natural infection. Higher arrival titre to all agents were sparing for subsequent disease risk. It was suggested that 'Pasteurella hemolytica' and bovine viral diarrhoea virus were causally related to UBRD because change in titre was associated with increased UBRD risk in this or other studies. For ' Hemophilus somnus' and bovine corona virus, no evidence existed that infection was associated with increased risk of UBRD treatment. Animals treated for UBRD late in the study period tended to show little or no evidence of exposure to 'Hemophilus somnus'. As this was not observed for 'Pasteurella hemolytica' titres, this suggested that exposure to 'Hemophilus somnus' was inhibited in animals receiving additional antimicrobials for UBRD treatment. The conclusion was drawn that 'Hemophilus somnus' and bovine corona virus were not causally related to UBRD occurrence. Higher arrival titres 'Hemophilus somnus' and bovine corona virus may indicate a functioning immune system in these calves, rather than indicating that titres are protective against the specific organism causing subsequent disease.