Three parallel lines of inquiry have been in progress on the investigation of the microsomal enzymes of cholesterol biosynthesis. First, the microsomal enzymes are being isolated and purified. Second, the enzymes are being recombined into partially-reconstituted systems that catalyze segments of the sterol biosynthetic pathway. Third, the relationship of microsomal electron transport to the microsomal oxidases of cholesterol biosynthesis is under study. A fourth line of work on the control of cholesterol biosynthesis has just been initiated. Current work will complete the isolation and purification of microsomal enzymes that are involved in more than one-half of the steps from lanosterol to cholesterol. Reconstitution work has been made possible by recent success in developing a fascile assay for methyl sterol oxidase and a means to stabilize the easily-denatured multienzymic system. Microsomal electron transport from alpha-NADH to methyl sterol oxidase now makes investigation of the stoichiometry of oxidation possible. Control of the microsomal enzymes of cholesterol biosynthesis appears to be related to both amount of enzyme and the presence of small-molecular-weight effector substances. Investigation of the latter is underway with microsomes from both liver and Morris hepatomas.