SummaryLymphoblastoid cell lines, infected and transformedin vitroby a Moroccan stock ofTheileria annulata, infected and immunized susceptible taurine cattle, at cell doses of 108, 106, 104and 102, regardless of whether the recipients were Bo LA matched or mismatched to the donor cell line. The MHC relationship between the cell line and recipient did affect the severity of the clinical response to cell line immunization which may reflect differences in the specific priming of the immune response. At the highest cell doses the BoLA–mismatched recipients reacted more severely than the BoLA–matched. This study shows that, unlike the closely related parasiteT. parva, there is no histocompatibility barrier to immunization usingT. annulata–infectedcell lines which could be achieved with as few as 102allogeneic infected cells. The role of MHC compatibility between cell line and recipient in the priming of a protective immune response is disc