In vitro studies were conducted to examine the metabolism of methionine (Met) and threonine (Thr) using mixed ruminal bacteria (B), mixed ruminal protozoa (P), and a combination of these two (BP). Rumen microorganisms were collected from fistulated goats fed with lucerne cubes (Medicago sativa) and a concentrate mixture twice a day. Microbial suspensions were anaerobically incubated with or without 1 mM each of the substrates at 39 degrees C for 12h. Met, Thr and their related amino compounds in both the supernatants and microbial hydrolyzates of the incubation were analyzed by HPLC. Met was degraded by 58.7, 22.1, and 67.3% as a whole in B, P, and BP suspensions, respectively, during 12h incubation. In the case of Thr, these values were 67.3, 33.4, and 76.2% in B, P, and BP, respectively. Met was catabolized by all of the three microbial suspensions to methionine sulfoxide and 2-aminobutyric acid. Catabolism of Thr by B and BP resulted in the production of glycine and 2-aminobutyric acid, while P produced only 2-aminobutyric acid. From these results, the existence of diverse catabolic routes of Met and Thr in rumen microorganisms was indicated.