An alkaline 5'-phosphodiesterase (5'-PDE) from barley (Hordeum distichum) malt sprouts was partially purified by thermal treatment and acetone precipitation to diminish phosphomonoesterase (PME) activity. 5'-PDE was purified 40-fold to a specific activity of 30 U mg(-1) protein with a final yield of about 32%. With synthetic substrate, the enzyme had an optimum pH of 8.9, maximum activity at 70degreesC over 10 min, and a Km of 0.26 mM. The partially purified enzyme was activated by 10 mM Mg2+ up to 168% of the original activity, while Zn2+, Mn2+ and Cu2+ ions, chelating agent (EDTA) and NaN3 (1 - 10 mM), and 5'-ribonucleotides (1 - 5 mM) were inhibitory. Final enzyme preparation was stable over 8 d at 4degreesC), at 70degreesC for up to 120 min and without loss of activity over 90 d at - 18degreesC.