Sera of 20 guinea-pigs experimentally infected with Trypanosoma evansi were obtained in order to compare the efficacy of gel diffusion, indirect immunofluorescence and agglutination tests, to detect antibodies to T. evansi. The fluorescent antibody test was positive in six (6) animals and the antibody titres were very low (1:4 to 1:16). The agglutination test detected trypanosomal antibodies in sera one (1) week after infection. After two (2) weeks all animals were positive with high titres (1:8.000 to 1:250.000). Agglutination was inhibited when sera were treated with 2-Mercapto-etanol. This fact suggests that IgM is the principal class of antibodies in sera of infected guinea-pigs. Precipitating antibodies were not detected during the course of infection.