In the present study, the enzyme acetylcholinesterase (AChE) from filarial parasites was identified in sera from humans infected with Onchocerca volvulus as well as in Mastomys natalensis infected with Brugia pahangi. The enzyme was present in immune complexes precipitated with cold 4% polyethylene glycol. The infected sera showed 3-4 times more AChE activity than did normal sera, and enzyme activity could be demonstrated in 5% polyacrylamide gels by specific staining. The enzyme from infected serum showed 3 times more activity when acetylthiocholine was used as the substrate as compared with butyrylthiocholine, whereas the enzyme activity present in normal serum was low and did not show this substrate specificity. Immunoprecipitation assays confirmed the presence of anti-AChE antibodies in the infected serum. The enzyme was further analysed by enzyme-linked immunosorbent assay and immunoblotting with rabbit antibodies to B. malayi AChE. Immunoblotting of the B. pahangi-infected serum revealed two closely located bands at about 200 kDa and one 95-kDa band, whereas in O. volvulus-infected serum, only one specific band was observed at about 200 kDa. The identification of parasite AChE may be particularly useful for diagnosis of the disease or for the study of the involvement of this enzyme in the host-parasite relationship.