Five of a group of six calves were inoculated with Mycobacterium bovis. Two more uninoculated calves were introduced to the group 84 days later. All the inoculated calves were subsequently shown to be excreting M bovis in nasal mucus. The uninoculated calf in the initial group of six became infected and subsequently excreted M bovis. The two uninoculated calves which were introduced later did not become infected. It was concluded that contact with nasal mucus from the infected cattle resulted in infection of the uninoculated calf and that the density of accommodation of animals excreting M bovis was an important factor in transmission of the disease.