Following the addition of 14C-labelled casein to mixed rumen bacteria at 39 degrees, some radioactivity was adsorbed to the bacteria before the casein was hydrolysed. At 0 degrees, the rate of hydrolysis was greatly diminished but adsorption still occurred, and this enabled a study of the adsorption mechanism to be made. The adsorption of 14C-labelled casein to rumen bacteria was a saturable process. The maximum binding capacity was about 10 micrograms 14C-labelled casein/mg bacterial protein. The ability of bacteria to adsorb 14C-labelled casein was abolished when they had been boiled for 5 min. Boiling caused the release of material from the bacteria which rendered some undigested protein soluble in 50 g trichloracetic acid/l. Adsorbed 14C-labelled casein could be partly displaced by the addition of Triton X100 or an excess of unlabelled casein, or by boiling, or by removal of capsular material by blending. Adsorbed 14C-labelled haemoglobin could similarly be displaced by an excess of cold casein. When an excess of casein was added to bacteria to which glucose-6-phosphate dehydrogenase (EC I.I.I.49) and glucosephosphate isomerase (EC 5.3.I.9) had been adsorbed, little active enzyme was displaced. The susceptibility of different 14C-labelled proteins to hydrolysis corresponded to their relative adsorption affinities. The pattern of sensitivity to inhibitors of the adsorption mechanism was the same as that for the inhibition of the bacterial hydrolysis of 14C-labelled casein, and the synthetic substrates leucine p-nitroanilide and benzoyl arginine p-nitroanilide. It was concluded that the adsorption site and the catalytic site for proteolysis by rumen bacteria are probably identical and so not likely to be subject to independent manipulation.