Ornithine aminotransferase (l-ornithine:2-oxoacid aminotransferase) (EC 2.6.1.13) was localized in the livers of rats fed diets differing in protein content with an indirect fluorescent antibody technique. The ornithine aminotransferase specific fluorescence in livers from rats fed a 60% protein diet was distributed throughout the cytoplasm of hepatic cells, and numerous fluorescent cells were observed throughout the lobules. Stained hepatic cells of rats fed a standard laboratory diet (24% protein) appeared to be less numerous and the fluorescence less intense than in rats fed the 60% diet. Livers from rats fed the 0% protein diet showed no specific fluorescence. The identification of ornithine aminotransferase in situ with the fluorescent antibody technique is compatible with measurements of the activity of enzyme extracted from the livers of rats fed similar diets.