Larvae of the trematode, Fasciola hepatica, dissected from the livers of intact mice at 12 to 14 days of infection, bind anti-immunoglobulins in vitro. This is not so for larvae harvested from the livers of infected hypothymic nude (nu/nu) mice. When attempts were made to confirm the presence of surface Ig using lactoperoxidase-catalysed radioiodination and analysis by SDS-polyacrylamide gel electrophoresis, proteolysis at the time of solubilization markedly affected the profiles obtained. Provided protease inhibitors were present during processing, a -like Ig heavy chain and a -like Ig heavy chain could be demonstrated on larvae from nu/+ mice but not on larvae from nu/nu mice.