Properties of the extracellular amylase produced by the psychrotrophic bacterium, Arthrobacter psychrolactophilus, were determined for crude preparations and purified enzyme. The hydrolysis of soluble starch by concentrated crude preparations was found to be a nonlinear function of time at 30 and 40 degrees C. Concentrates of supernatant fractions incubated without substrate exhibited poor stability at 30, 40, or 50 degrees C, with 87% inactivation after 21 h at 30 degrees C, 45% inactivation after 40 min at 40 degrees C and 90% inactivation after 10 min at 50 degrees C. Proteases known to be present in crude preparations had a temperature optimum of 50 degrees C, but accounted for a small fraction of thermal instability. Inactivation at 30, 40, or 50 degrees C was not slowed by adding 20 mg/ml bovine serum albumin or protease inhibitor cocktail to the preparations or the assays to protect against proteases. Purified amylase preparations were almost as thermally sensitive in the absence of substrate as crude preparations. The temperature optimum of the amylase in short incubations with Sigma Infinity Amylase Reagent was about 50 degrees C, and the amylase required Ca(+2) for activity. The optimal pH for activity was 5.0-9.0 on soluble starch (30 degrees C), and the amylase exhibited a K (m) with 4-nitrophenyl-alpha-D-maltoheptaoside-4,6-O-ethylidene of 120 microM at 22 degrees C. The amylase in crude concentrates initially hydrolyzed raw starch at 30 degrees C at about the same rate as an equal number of units of barley alpha-amylase, but lost most of its activity after only a few hours.