A bioprocess employing Microbacterium sp. MA 5614 was developed for the production of the MK-0476 (S)hydroxy ester by asymmetric bioreduction of the corresponding keto ester. The cysteinyl leukotriene 1 receptor antagonist MK-0476 [3-[(1S)-[3(E)-[2-(7-chloroquinolinyl)ethenyl]phenyl]-3- (acetylphenyl)-propylthiol]-2(S)-methylpropanoic acid] is currently undergoing clinical evaluation for the treatment of asthma. Optimization of the bioconversion medium composition and the time of keto ester addition, as well as the development of a keto ester feeding strategy yielded improvements in the bioreduction rate and final (S)-hydroxy ester titer, by 20- and 25-fold respectively. This asymmetric bioreduction process supported the production of preparative quantities of (S)-hydroxy ester with an enantiomeric excess greater than 95%.