In a small proportion of persons infected with Trypanosoma rhodesiense the parasitaemia is so slight or so intermittent that a positive diagnosis cannot be made by the routine parasitological examination of blood and glandular fluid. By the time the infection is mature enough to be diagnosed by these methods these persons may not readily have access to a diagnostic centre. Some of these infections could be diagnosed earlier by repeated intensive examinations of blood and cerebrospinal fluid, and by animal inoculation tests. A stimulus for investigators to make these examinations could be provided by the results of a simple, rapid, and highly specific immunological screening test that could be made concurrently with the first parasitological examination of suspected cases. The results of two such tests for detecting circulating trypanosomal antibodies are described. One of the tests, the latex agglutination test, is potentially useful as a screening test for persons whose first parasitological examination is negative but whose clinical or social background suggests that they may be infected. This test could find wide application in hospitals and dispensaries, and in the field. Studies indicate that there is little value in testing suspected cases of trypanosomiasis for heterophile antibodies.