Clinical and serum antibody profiles were studied during oral Listeria monocytogenes infection of calves. No clinical signs, except for pyrexia with mild diarrhoea and staggering gait, were observed in the infected calves. Specific antibodies to listeriolysin O (LLO) appeared as early as day 8 of an oral infection and peaked by days 16-32 of infection. Antibodies to LLO were observed to persist over the period of 126 days observed in the study. LLO being a major virulence factor and capable of inducing a humoral response could therefore be used as an antigen for development of an immunoassay for diagnosis of Listeria monocytogenes infections in animals.