This work is part of a project that is currently applying microbiology to certain aspects of the multiple sclerosis problem. The present comparative studies were carried out in connection with atherosclerosis because this disease, like multiple sclerosis, involves deranged lipid metabolism. Twenty-one soil actinomycetes and a bacterium were isolated from agar streak plates containing homogenized human aorta as the source of nitrogen, carbon, and energy. These organisms included forms capable of digesting and growing on the ether-extractable lipids from human atherosclerotic aorta plaques, cholesterol, cholesterol esters, glycerides, phospholipids, bovine white matter and other related materials. Two actinomycetes, isolated in other studies and capable of demyclinating bovine spinal cord in witro, produced lipoproteinase(s) which hydrolyzed lipemic serum obtained from dogs after a fatty meal.