Efrotomycin is a narrow spectrum antibiotic. Among the genera tested for susceptibility in vitro it is most active against isolates of Moraxella, Pasteurella, Yersinia, Haeniophilus, Streptococcus and Corynebacterium. The drug is as active by oral administration as by the subcutaneous route. Blood levels rise rapidly to high concentrations, after oral dosing, and are prolonged. Two peaks occur which may indicate biliary excretion and reabsorption. Urinary excretion is minimal. The high blood concentrations explain, in part, the in vivo activity against pathogens such as Bordetella bronchiseprica which are relatively insensitive in vitro. Oral activity of efrotomycin is an advantage over the related antibiotics, X-5108 and mocimycin.