D-1-Amino-2-propanol:NAD+ oxidoreductase activity, which catalyzes the second step in a pathway wherein L-threonine is converted to D-1-amino-2-propanol via the intermediate formation of aminoacetone, has been purified 500-fold from Escherichia coli K-12. Although the enzyme catalyzes the oxidation of certain diols as well as 1-amino-2-propanol, it is completely specific for the D-isomer of the amino alcohol and for NAD+. Two molecular forms (designated Form L and Form S) of the oxidoreductase, both of which are catalytically active, have been separated by gel filtration on Sephadex G-200; apparently, Form L is converted to Form S by dissociation (Form L leads to Form S). Molecular weight determinations indicate that the two forms of the enzyme are different not only in size but also in shape; Form L apparently is an asymmetric tetramer of Form S. The two molecular species have similar catalytic properties. Both exhibit the same pH optimum of 8.6, have nearly identical apparent Km values for substrate and cosubstrate, are equally sensitive to inhibition by p-mercuribenzoate and N-ethylmaleimide, and show the same specificity for cosubstrate. Neither form of the enzyme has an absolute requirement for added thiol compounds or divalent metal ions.