SummaryUsing mice harbouring early Fasciola hepatica infections, six monoclonal antibodies were prepared against a tegumental antigen present in T1 granules and glycocalyx of flukes. Blocking tests indicated that all monoclonals bound the same T1 epitope (or epitopes in close proximity on the antigen molecule), but this was not the determinant recognized by sheep and cattle. Localization of antibody binding at light and electron microscope levels showed that T1‐type antigen also occurred in metacercarial tegument and in glycocalyx of gut cells and excretory ducts in juvenile and adult flukes. This indicates that the natural host‐antibody response to F. hepatica may be to one antigen early in the infection. Protein A‐gold labelling of monoclonal treated fluke sections revealed that the epitope was probably a polypeptide, unmodified by glycosylation in Golgi bodies. When isolated by immunoadsorption and separated electrophoreti‐cally under reducing conditions T1‐type antigen was found to consist of a polypeptide mol. wt. 50000, possibly linked to smaller entities mol. wt. 25–40 000. Tissue‐specific variations in the antigen molecule might be conferred by linkage of different polypeptides or carbohydrate side‐chains to an antigenic core polypeptide. A component of T1‐type antigen was found to have mol. wt. of 25000, possibly resembling a polypeptide of mol. wt. 24000 from Schistosoma