Studies designed to assess the role of acyl-CoA:sn-glycerol-3-P acyltransferase (AT) in regulating both the structure and function of lipids in mammalian cell and organelle membranes are planned. We have shown earlier that mammalian cell mitochondria house an AT that is distinguished from a similar microsomal enzyme by its substrate and positional specificity. These unique properties suggest that the mitochondrial AT may play an important role in directing the asymmetric placement of saturated and unsaturated fatty acids in cellular glycerolipids. It has been known for some timme that the positional distribution of fatty acids in lecithin isolated from Ehrlich ascites tumor cells (EAC) was considerably more random than that of normal tissues. We recently reported that EAC were devoid of mitochondrial AT, although the microsomal enzyme was present at levels comparable to most mammalian tissues. We intend to compare mitochondrial AT activity and the fatty acid distribution in the glycerolipids of whole cells and organelles in cultured cells to their transformed counterparts. These studies are intended to determine the generality (or lack thereof) of the absence of mitochondrial AT in tumor cells and to decide if a correlation exists between the absence (or altered specificity) of the mitochondrial enzyme and the randomness of fatty acids in membrane glycerolipids. We also intend to examine the possibility that cellular differentiation is a prerequisite for the appearance of the mitochondrial enzyme.