The cyclic nucleotide phosphodiesterase secreted by Physarum polycephalum plasmodium into extracellular medium has been partially purified by DEAE cellulose chromatography, ultrafiltration, and HPLC. The results obtained by gel filtration, HPLC, electrophoresis, and isoelectric focusing suggest that, the native enzyme in solution is a monomer with a molecular mass of about 90 kDa and pI in the range 3.6 - 4.0. The Km values were estimated to be about 0.9 mM and 7.7 mM, respectively, and Vm for both substrates were similar (up to several thousand micromoles of cAMP hydrolyzed/hour per mg of enzyme). The partially purified enzyme was shown to be extremely stable. It did not lose the activity after heat treatment at 100 degrees C during 30 min. The enzyme was active in the presence of 1% SDS, but it was fully inactivated under the same conditions in the presence of beta-mercaptoethanol. The properties of the phosphodiesterase from Physarum polycephalum are discussed.