1. Mitochondria, inner and outer mitochondrial membranes and microsomes were 
isolated and purified from pig heart. Their lipid composition and protein 
components were studied. 2. The fatty acid distribution in the main 
phospholipids seemed specific rather of a given phospholipid and not of one type 
of membrane. 3. Inner mitochondrial membranes were characterized by a high 
content in cardiolipin and a very low level of triglycerides together with a 
high degree of unsaturation and C18 acids. Gel electrophoresis revealed 13 
different polypeptide subunits of which 5 were major ranging in molecular 
weights from 10000 to 215000. 4. In outer mitochondrial membranes, total lipid, 
phosphatidylcholine, phosphatidylinositol, plasmologen and triglyceride contents 
were much higher than in inner membranes. Fatty acids of phospholipids were 
mostly saturated and the polypeptide pattern showed 12 components, of which 4 
were major of mol. wt 75000, 60000, 20000 and below 10000. 5. Compared to outer 
membrane, microsomes exhibited a much higher cholesterol content and markedly 
different protein profiles. They contained significant amounts of cardiolipin 
and phosphatidylserine, this latter phospholipid being exclusively located in 
microsomes. However odd similarities were observed in some lipid components of 
microsomes and inner mitochondrial membranes, but fatty acids were more 
saturated in microsomes and electrophoretic profiles of protein components 
appeared very different and revealed components of high mol. wt.