L-allo-Threonine aldolase (L-allo-threonine acetaldehyde-lyase), which exhibited specificity for L-allo-threonine but not for L-threonine, was purified from a cell-free extract of Aeromonas jandaei DK-39. The purified enzyme catalyzed the aldol cleavage reaction of L-allo-threonine (K(m) = 1.45 mM, Vmax = 45.2 mumol min-1 mg-1). The activity of the enzyme was inhibited by carbonyl reagents, which suggests that pyridoxal-5'-phosphate participates in the enzymatic reaction. The enzyme does not act on either L-serine or L-threonine, and thus it can be distinguished from serine hydroxy-methyltransferase (L-serine:tetrahydrofolate 5,10-hydroxy-methyltransferase, EC 2.1.2.1) or L-threonine aldolase (EC 4.1.2.5).