An incubation mixture of TDP-D-glucose and a purified enzyme extract from E. coli B containing TDP-D-glucose 4,6-hydro-lyase was treated with NaB(3)H(4). Hydrolysis of the separated sugar nucleotide fraction yielded D-galactose-4-(3)H but no 3H-labelled D-glucose. This indicates the presence of TDP-4-keto-D-glucose as an intermediate in the enzymatic conversion of TDP-D-glucose to TDP-L-rhamnose. The stereospecifity of the NaB(3)H(4) reduction can only be explained if TDP-4-keto-D-glucose exists as an enzyme bound complex.