After introducing the basic problems of trypanosomiasis of domestic stock in tropical Africa, a review is given of the historical development of research, trypanosome nomenclature, some immunological aspects of the disease, the diagnostic techniques in current use, and some of the trypanosome handling methods. The development, of techniques for the serological study of Trypanosoma congolense, and their subsequent use in experiments studying antigenic variation of these trypanosomes and the immune response they induce in infected cattle, is described. A serological examination of populations of T.congolense, before and after cyclical passage through the tsetse fly(Glossina morsitans), is also undertaken using the same techniques. The agglutination test is used to examine the antigenic relationship of geographically distinct isolates of the brucei sub -group trypanosomes. The efficacy of infection with virulent trypanosomes followed by chemotherapy as an immunization procedure, and the value of an indirect fluorescent antibody test as a diagnostic aid, in cattle infected with pathogenic trypanosomes, are subsequently studied. A discussion follows with particular reference to the advances made in this study, the types of acquired immune mechanism which develop in cattle infected with pathogenic trypanosomes, the feasibility of immunological control of trypanosomiasis, and the use of serological tests in diagnosis.