The role of the mesosome in M.lysodeikticus has been studied using both biochemical and electron-microscopic techniques. During the developement of membrane fractionation procedures, unexpected variations in the specific activity of succinate dehydrogenase (EC 1.3.99.1) were observed in similar membrane preparations. Since this enzyme is an important membrane "marker", a detailed study of the factors affecting its activity was considered an essential preliminary investigation. The membrane-bound enzyme was inactivated at temperatures above 1
