The results presented below show that the dividing and adult forms of T. lewisi share common antigens and indicate that the persistence of adult forms in the circulation of rats immune to reinfection is not due to a change in surface antigens as has been postulated. Using a rabbit anti-rat immunoglobulin serum, the presence of rat immunoglobulins on the surface of adult trypanosomes could be demonstrated. These immunoglobulins were not complement-fixing or opsonic. It is suggested that these immunoglobulins are responsible for the persistence of the adult forms in the circulation of the rat.