Phospholipase B inhibitor was found in the autolyzate of yeast cells, Torulaspora delbrueckii. The inhibitor was purified to homogeneity by ethanol precipitation, gel filtration with Sephadex G-10, ion-exchange chromatography with DEAE-Sephacel, and gel filtration with Asahipak GS-320. On thin-layer chromatography the purified inhibitor was detected with the Hanes-Isherwood reagent, which is used to detect phosphorus. The activity of the inhibitor was not affected by heat treatment at 100 degrees C for 1 h. Heating at 100 degrees C for 1 h in 1 M HCl and 1 M NaOH lowered activity to 76 and 80% of the original values, respectively, but heating at 110 degrees C for 24 h in 6 M HCl completely abolished activity. The inhibitor was highly soluble in water, but practically insoluble in alcohol, acetone, ether, and chloroform. The degree of inhibition of enzyme activity was not proportional to the concentration of inhibitor. The inhibitor inhibited both membrane-bound and water-soluble phospholipase B activity from T. delbrueckii at the same level; however, the inhibitor did not inhibit the activity of phospholipase A2 from snake venom (Naja naja).