A purified and partially characterized novel NADP(+)-dependent oxidoreductase from Clostridium tyrobutyricum DSM 1460 was applied for the preparative reduction of several 3-oxo acids to (S)-3-hydroxy acids. (R)-3-Hydroxybutyrate was prepared by the same enzyme selectively dehydrogenating the S enantiomer of (R,S)-3-hydroxybutyrate. The enantiomeric purity of the (S)- and (R)-3-hydroxy acids was at least 98% enantiomeric excess (e.e.). NADPH for reductions and NADP(+) for dehydrogenations were regenerated by applying artificial mediator accepting pyridine nucleotide oxidoreductases in the form of a crude extract of C. themoaceticum cells. For NADP(+) regeneration also the system 2-oxoglutarate/glutamate dehydrogenase was used for comparison. Instead of the purified (S)-3-hydroxycarboxylate oxidoreductase, resting cells of C. tyrobutyricum were also applied for reductions and dehydrogenations with substrate concentrations of 200-400 mM leading to products with e.e. values above 96%.