An analysis of Hymenolepis diminuta specific antigens in infected rat faeces was carried out. Using a capture type antibody sandwich ELISA assay based on a hyperimmune rabbit anti-worm somatic antisera it was demonstrated that, although antigen was present in faeces before patency, the onset of egg production led to a sharp increase in the levels of parasite antigen in the faeces. Levels of antigen in host faeces were independent of worm burden. Parasite eggs did not contribute significantly to faecal antigen levels. Western blot analysis indicated a number of highly specific antigens at around Mr 69,000, Mr 37,000, Mr 50,000 and Mr 27,000 with a low molecular weight smear at between Mr 30,000 and Mr 34,000 present in the faeces of H. diminuta infected rats. Some cross reaction occurred with an antigen of around Mr 66,000 in the faeces of non H. diminuta infected rodents. Antibody activity against this antigen was removed by affinity adsorption of the antibody solution against normal rat faeces.