Decanamine hydrochloride (a C10 primary amine), N-methylundecanamine (a C11 secondary amine) and N, N-dimethyldodecanamine (a C12 tertiary amine) were tested for their effects upon in vitro cellulose digestibility by unadapted ruminal microorganisms. The three amines exhibited similar patterns of inhibition of cellulose digestibility in 48-hr in vitro incubations. Cellulose digestion was depressed to 97, 89, 31, 11 and 12% of the control value by 5, 10, 25, 50 and 100 ppm of the amines, respectively. Total volatile fatty acid production was depressed to 89, 85, 61, 40 and 32% of the control value by 5, 10, 25, 50 and 100 ppm of the amines, respectively. Propionate production was inhibited to a greater extent than was acetate production, and acetate to propionate ratios increased in the presence of the amines. The results demonstrate that these primary, secondary and tertiary amines inhibit those microorganisms which digest cellulose in the rumen.