A method for microbial protein determination based on Lowry's assay was devised. Differential centrifiguration was used to separate bacterial cells from rumen liquor. Proteins from cells were released by suspending in .25 N sodium hydroxide and heating in boiling water bath for 10 min. Protein by the Foin phenol method. The method is simple, accurate, reproducible, sensitive, specific, brief, and can be performed with small amount of rumen liquor (5 ml). A large number of samples can be handled conveniently.