The influence of administering sheep red cells (SRC) as antigens, either after or before a trypanosome challenge, on parasitaemia profile and antibody response was assessed in albino Wistar rats. High levels of parasitaemia associated with significantly depressed antibody response and packed cell volume (PCV) values were observed when trypanosome challenge preceded antigen stimulation. In contrast, a clear delay in the onset and development of parasitaemia occurred when antigen priming preceded trypanosome challenge. At the beginning, PCV values and antibody response to the antigen were in the range of levels found in control rats. However, as infection progressed, parasitaemia rose and significant immunological hyporesponsiveness developed which at least reached levels found in rats that had received trypanosome challenge prior to antigen stimulation. These findings should be taken into consideration when evaluating serological tests used for assessing responses to specific vaccinations, or for the diagnosis of infections based on rising antibody titres in the host.