Monoclonal antibodies (mAbs) were produced against soluble metabolic products (excretory/secretory; ES) of the liver fluke, Fasciola gigantica. The ES antigen was obtained from spent culture medium in which the adult flukes had been maintained in vitro. From two cell fusions, the mAbs produces were exclusively associated with either IgG or IgM isotypes. When screened against a panel of homologous and heterologous parasite antigens by indirect ELISA, two mAbs were highly specific for F. gigantica. The remainder cross-reacted extensively with other parasites. Results from immunoblotting and immunofluorescence exhibited two patterns of reactivity. The first group of mAbs which recognized the multiple bands between > 14.4-27.5 kDa gave extremely bright fluorescence over all major muscular systems, vitelline gland, testes and intestinal caeca. The second group which reacted with the 185 kDa band showed a bright fluorescence over a thinner muscular layer underlying the tegument, intestinal caeca and testes.