Adult Strongylus vulgaris, collected from the caecum of infected horses and embedded in paraplast using standard methods, were sectioned for immunohistochemistry (IHC) studies. Antibodies were raised in rabbit against the excretory–secretory product (ESP) and against two constituent protein bands (28–30 kDa). The use of sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS-PAGE), enzyme-linked immunosorbent assay (ELISA) and immunoblotting indicated the immunogenicity of ESP and of the subunits (28–30 kDa). In ELISA, both rabbit hyperimmune sera recognized the ESP and (28–30 kDa) bands consistently and strongly. Both hyperimmune sera recognized most ESP subunits (80, 60, 54, 42, 35, 30, 20 and 15 kDa) in immunoblots. IHC, using light microscopy, suggested that the following worm tissues reacted strongly and positively with both antisera: amphids, tooth core, intestine, excretory gland and ducts, and hypodermis. Thus, either these are antigen-producing tissues, or antigens sharing common epitopes occur in them. The following tissues reacted weakly: body cuticle, buccal capsule cuticle, oesophagus, and also somatic muscle (non-contractile portion) perhaps due to diffusion of antigen from adjacent tissues. Preimmune serum gave a negative reaction with most worm tissues.