An in vitro experiment was conducted to test the ability of mixed rumen bacteria (B), protozoa (P), and their mixture (BP) to utilize the oxidized forms of methionine (Met) e.g., methionine sulfoxide (MSO), methionine sulfone (MSO(2)). Rumen contents were collected from fistulated goats to prepare the microbial suspensions and were anaerobically incubated at 39 degrees C for 12 h with or without MSO (1 mM) or MSO(2) (1 mM) as a substrate. Met and other related compounds produced in both the supernatants and hydrolyzates of the incubation were analyzed by HPLC. During 6- and 12-h incubation periods, MSO disappeared by 28.3 and 42.0%, 0.0 and 0.0%, and 40.6 and 62.4% in B, P, and BP suspensions, respectively. Rumen bacteria and the mixture of rumen bacteria and protozoa were capable to reduce MSO to Met, and the production of Met from MSO in BP (156.6 and 196.1 micromol/g MN) was about 17.3 and 14.1% higher than that in B alone (133.5 and 171.9 micromol/g MN) during 6- and 12-h incubations, respectively. On the other hand, mixed rumen protozoa were unable to utilize MSO. Other metabolites produced from MSO were found to be MSO(2) and 2-aminobutyric acid (2AB) in B and BP. MSO(2) as a substrate remained without diminution in all-microbial suspensions. It was concluded that B, P, and BP cannot utilize MSO(2); but MSO can be utilized by B and BP for producing Met.