A procedure for the purification of 3-hydroxy-3-methylglutaryl-coenzyme A reductase [mevalonate: NADP+ oxidoreductase (CoA-acylating); EC 22.214.171.124] solubilized from rat liver microsomes is reported. This enzyme has a specific activity of 9,000-10,000 nmol of mevalonate formed per min/mg of protein. This represents a 4100-fold purification over the activity in microsomes, and a specific activity that is approximately 20-fold greater than the highest previously reported value. The enzyme is judged to be homogeneous on the basis of sodium dodecyl sulfate/polyacrylamide disc gel electrophoresis, polyacrylamide disc gel electrophoresis, and immunoanalysis. Data are also presented that indicate the separation of enzymatically active and inactive species of 3-hydroxy-3-methylglutaryl-coenzyme A reductase on affinity chromatography on a coenzyme A column.