Mevalonate kinase can be conveniently assayed by coupling to two other reactions and monitoring the consumption of NADH optically at 340 nm. No mevalonate kinase was detected in crude extracts of Saccharomyces cerevisiae strains, however, because of background interference measured in the absence of mevalonate. A strain of S. cerevisiae over-expressing mevalonate kinase was used to establish conditions for reduction of background interference. This method has been successfully applied to S. cerevisiae strains containing a wild type level of mevalonate kinase.