A monomeric 17-kDa hemolysin designated as eryngeolysin was isolated from fresh fruiting bodies of the mushroom Pleurotus eryngii, using a protocol that involved gel filtration on Superdex 75, ion exchange chromatography on Mono Q and gel filtration on Superdex 75. Its N-terminal sequence demonstrated striking homology to that of its counterparts ostreolysin from the oyster mushroom Pleurotus ostreatus and aegerolysin from the mushroom Agrocybe cylindracea. Its hemolytic activity was unaffected over the pH range 4.0-12.0, but no activity was observed at pH 13 and at and below pH 2. The hemolysin was stable between 0 and 30 degrees C. At 40 degrees C, only residual activity was detectable. At and above 50f degrees C, activity was indiscernible. Eryngeolysin exhibited cytotoxicity toward leukemia (L1210) cells but not toward fungi. The hemolysin was inactivated by treatment with trypsin. It exhibited antibacterial activity against Bacillus sp. but not against other species. It inhibited basal as well as ConA-stimulated mitogenic response of murine splenocytes. N-Glycolyneuraminic acid was the only sugar capable of inhibiting the hemolytic activity. Eryngeolysin-induced hemolysis was osmotically protected by polyethylene glycol (PEG) 10000 with a mean hydrated diameter dose to 9.3 nm. However, no protection was offered by PEG 10000 to the anti-mitogenic and antiproliferative activities of eryngeolysin. The susceptibility of erythrocytes from different classes of vertebrates to eryngeolysin was mammalian > avian > reptilian > piscine.