Groups of lambs were either slaughtered at weekly intervals between November and June, or regularly sampled over this period. The number of isolations ofPasteurella haemolyticaandMycoplasmasp. from the lungs increased during the later part of two seasons, as did the incidence of lesions typical of enzootic pneumonia.P. haemolyticawas frequently isolated after February from nasal swabs in spite of the presence of serum antibody titres against this organism. Serum antibody titre to two adenoviruses were found to fall to a low level, one in January, the other in May, and then to rise again. Virus isolations were made at the time the homologous antibody titres were lowest.