Adult worms of Spirometra plerocercoids (spargana) were dissected out from the tissues of naturally infected snakes (natrix trigria lateralia). Fresh spargana were incubated in distilled water and their excretory/secretory (ES) products were collected. Proteinse activity of the ES products was assayed with a synthetic fluorescent substrate, carbobenzoxy-phenylalanyl-arginyl-7-amino-4-trifluoromethylcoumarin. By means of DEAE-Trisacryl M ion-exchange chromatography and thiolpropyl-sepharose affinity chromatography, the proteinase in the ESP resulted in a 15.6-fold purification. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) of the purified materials revealed a single 28 kDa band, consistent with the apparent native molecular weight (gel filtration chromatography) of approximately 28 kDa. The proteinase purified from the ES products were examined for various biochemical characteristics : 1) its inhibitor profile indicated it belonged to cysteine endopeptidases; 2) it exhibited identical pH curves with optimal at pH 5.5 and a 50% activity range from pH 3.7∼7.1; 3) it completely degraded collagen chains to three identical products; and 4) it showed only minor activity toward hemoglobin. These results indicate that the proteinase may be involved in the parasite nutrition acquisition and/or penetration and lysis of host tissues.