Triton X-100 and Lubrol PX at similar concentrations extracted more phospholipid than protein from rat liver and rabbit heart membranes, as compared with their original protein and phospholipid contents, respectively. All the types of membrane phospholipid were solubilized from rat liver membrane with the used non ionic detergents, but less sphingomyelin was extracted than phosphatidylcholine and phosphatidylethanolamine.