Low ionic strength acidic buffer, Sephadex G-200 and Benzamidine-Sepharose (BZ) gel chromatography, have been used for the partial purification of alveolar hydatid cyst (AHC) induced amyloid enhancing factor (AEF). BZ-gel bound AEF (AEF-BZ) demonstrated AEF activity in the mouse bioassay, proteolytic activity against Hide powder azure showed two major and three minor peptides on SDS-PAGE. Pretreatment of AEF-BZ with 10 mM phenylmethylsulphonyl fluoride or 20 mM p-chloromercuribenzoic acid completely abolished its bioactivity in vivo and proteolytic activity in vitro. Polyclonal anti-AEF antibody (AAA) was generated which on passive transfer into mice completely abolished the bioactivity of both casein-induced, or AHC-induced AEF. The AAA absorbed on Sepharose gel conjugated to normal mouse serum developed one common precipitin band between AE and AEF-positive sera from AHC-infected and old retired mice and in immunostaining it bound to the cytoplasmic granular components of a majority of splenic and peritoneal leucocytes from AHC-infected mice. In contrast, only a few normal mouse leucocytes showed positive staining. We suggest that AEF, in all probability, is a serine/thiol protease of leucocyte origin whose intracellular and humoral concentrations increase significantly during amyloidosis. The role of lysosomal proteases and anti-AEF antibody which has been successfully generated for the first time is discussed with reference to the origin of AEF and its presumed biological function in amyloidogenesis.