Abstract: Geranylgeraniol-18-hydroxylase, which catalyzes the C-18 hydroxylation of geranylgeraniol to form plaunotol, is extracted from Croton sublyratus using an extraction buffer and is purified using a differential centrifugation separation technique. The purified enzyme is used to convert geranylgeraniol to plaunotol, an anti-peptic ulcer agent. The enzymatic activity of the hydroxylase can be increased by a factor of about three by heating the enzyme in boiling water for about 30 minutes. The enzyme activity is also increased by adding a coenzyme, NADPH, which increases enzyme activity by a factor of about 2 or NADH, which increases activity by a factor of about 1.4. The pH optimum for hydroxylase enzyme activity is 5.0 The hydroxylase is a complex enzyme with a diameter of about 20 to 40 nm.