AbstractOne major reason for determining the amino acid sequence of proteins comprising the liver microsomal hydroxylase system is to be able to apply the sequence data together with methodologies for isolating peptide fragments toward elucidation of their orientation in the membrane. In earlier reports we described the isolation and determination of the complete primary structure of several liver microsomal cytochromesb, [1-4], P-450 [5-8], epoxide hydrolase [9], and cytochromeb5reductase [10]. More recently the complete amino acid sequence of microsomal stearyl-CoA desaturase [11]and the 60-kDa membrane esterase [12] was also reported. Although much is known about the molecular properties and pharmacology of monooxygenase proteins, only a limited number of studies have been done to elucidate the orientation of these proteins in the endoplasmic reticulum.