Three species-specific monoclonal antibody based antigen-ELISA (Ag- ELlSA) have been developed for diagnosis of the main livestock trypanosomiasis. Species-specificity and sensitivity of the Ag-ELISA are very satisfying. Used for the first time in 1991, in French Guyana, the Ag-ELISA gave surprising results which justified a re-evaluation of sensitivity and specificity. The evaluation was carried out at CIRAD-EMVT French Guyana and at ILRI, Kenya. The tests were evaluated with 2953 samples from cattle eventually infected by T. vivax only, 130 samples from cattle non infected, 160 samples from 4 calves and 338 samples from 2 sheep experimentally infected with an isolate of T. vivax from French Guyana, and, 70 samples from two sheep experimentally infected with an Asian clone of T. evansi. Results indicated that the sensitivity of the Ag-ELlSA T. vivax was very low, 2-4% in cattle or sheep infected; the sensitivity of the Ag-ELISA T. brucei was around 60% in sheep infected with T. evansi. Specificity of the tests was also very low. Tests developed with the use of these monoclonal antibodies or with monoclonal antibodies derived from this ones will never allow to set up sensitive and specific diagnosis. The use of these reagants should be stopped. For a sensitive and specific diagnosis based on Ag-ELISA, there is a need to develop new monoclonal antibodies; it is suggested that such project should be a collaborative research of several institutes