Abstract An inactive reduced UDP-galactose-4-epimerase complex, which is prepared by reduction with NaBH4 in the presence of substrates and is known to contain DPNH and UDP-hexoses in tightly bound form, is largely reoxidized and partially reactivated in the presence of 0.02 to 0.4 m cyclohexanone or cyclohexanol. Upon placing the epimerase-[4-β-3H]DPNH · UDP-hexose complex in the presence of 0.4 m cyclohexanone about 70% of the DPNH is reoxidized and about 40% of the catalytic activity is restored within 90 min at 27° and pH 8.5, while the major radioactive products are 3H2O and free tritiated DPN+, as well as enzyme-bound tritiated DPN+, but not tritiated cyclohexanol. There is no evidence that cyclohexanone is reduced, and cyclohexanol at similar concentrations causes a similar reactivation. It is probable that these compounds act by causing the tightly bound UDP-hexose molecules to be released from the reduced complex. The resultant epimerase · DPNH complex then undergoes spontaneous autoxidation and reactivation.