The relationship between ethanol consumption and the bioreduction of prochiral ketones in bakers’ yeast cells using ethanol as a reductant for NAD(P)H regeneration was examined and the efficiency (nu(sub)/nu(EtOH)) correlated to k(L) alpha. Ethanol was consumed at almost the same specific rate in the presence or absence of the ketones. In the reduction of ethyl acetoacetate catalyzed by NADPH-dependent oxidoreductase, the efficiency (nu(EA)/nu(ELOH)) decreased with increasing k(L) alpha and was about 0.5 at the actual operating condition of kLa of 100 h(-1). The reduction of acetol catalyzed by NADH-dependent oxidoreductase proceeded through the use of a portion of NADH in the regenerated NADH, and the efficiency (nu(acetot)/nu(EtOH)) had a maximum (1.9) at k(L) alpha of 20h(-1)