Peptic partial hydrolysates of a different hydrolysis degree were prepared from horse bean protein isolate and used for the plastein reaction with thermitase. Whilst unsoluble products were formed from partial hydrolysates of a hydrolysis degree above 60%, plastein gel could be obtained from a less intensely split basic material. By gel chromatography and by determination of the percentage which was unsoluble in 10% trichloroacetic acid, a proteolytic degradation could be made proof on the conditions of the plastein reaction. On addition of DL-methionine ethylesters to the partial hydrolysate an increasing incorporation of methionine into the plastein could be observed with increasing hydrolysis degree.