SUMMARYTwenty-four pigs experimentally infected with swine epizootic pneumonia (SEP) were divided equally into four groups. Three groups, 1 to 3, were treated with 25, 50, and 100 ppm tyamulin, an antibiotic of the diterpene group, respectively. The other group served as control.As a result, coughing was inhibited more effectively in groups 2 and 3 than in any other group. Feed demand was lower in rate in groups 2 and 3 than in any other group. Autopsy revealed much severer pneumonic changes in the control group and group 1 than in the other groups.It was assumed that 50 ppm or more of tyamulin might be effective for the spontaneous case of infection in the field.