A series of monoclonal antibodies has been produced which reacts with the intracellular macroschizont of Theileria annulata, and this series has been used to examine the level of antigenic diversity between and within stocks of the parasite in addition to species specificity within the genus. The majority of the monoclonal antibodies (six of eight) reacted with all stocks tested and four were species-specific. Two monoclonal antibodies detected variation between stocks and within stocks. The variation within a stock was manifest as only a proportion of the macroschizonts reacting with the monoclonal antibody. This was demonstrated as being due to the strain containing more than one antigenic type of parasite by cloning the infected lymphocyte cell line and showing that the resulting cloned cell lines were antigenically homogeneous. The variation between stocks allowed them to be divided into three groups on the basis of antigenic profile.