This study describes the effect of allogeneic immune responses on immunisation against Theileria annulata with a parasite infected cell line. The dynamics of lymphocyte populations during development of the immune response against the parasite after cell line immunisation and sporozoite challenge were investigated in peripheral blood, and lymph efferent from the lymph node draining the site of infection. T. annulata infected cell lines have been used as vaccines against tropical theileriosis in several countries. Inoculated animals produce a strong response against the allogeneic MHC antigens of the immunising cell line followed by an anti parasite response. There is evidence that immunity to the parasite wanes in the absence of challenge and reimmunisation is often recommended. However, the effect of pre-existing allogeneic responses generated after first immunisation with T. annulata infected cell lines upon development of immunity against the parasite at the time of reimmunisation is not known To investigate this, an allogeneic response was first generated in the animals followed by immunisation with T. annulata infected cell line of the same BoLA type. A mild allogeneic response generated by inoculation of uninfected leucocytes did not effect the development of immunity during cell line immunisation. However, a strong anti MHC response generated by skin grafting interfered with the development of a parasite specific immune response when the animals were immunised with 1x10⁶ cells. Effect of the allogeneic response was more marked when vaccination was carried out with a lower cell dose of 1x10⁴ cells where the development of immunity against T. annulata was completely blocked. These observations are of immediate importance in endemic areas where T. annulata infected cell lines are being used as vaccines to control the disease.