Abstract: A method and apparatus for determining the enzyme or enzymes in the human body which metabolize a particular drug. Microsomes are obtained from each of several donors. The microsome for one donor is reacted with a test drug and the quantity of metabolites produced is determined. The microsomes are similarly each reacted with the test drug and the quantity of metabolites produced for each microsome is determined to generate drug metabolism data. The drug metabolism data obtained is compared to reference data which indicates the activity of a select number of major enzymes in each of the donors. The reference data for each enzyme is separately tabulated. The enzyme responsible for the metabolism of the test drug is identified when the metabolism data correlates with the tabulated reference data for that enzyme.