The mode of action of Buparvaquone on Theileria sp. is not yet cleared, although studies indicate an effect on energy generation. Electron microscopical studies show progressive vacuolation of the cytoplasm as the principal lesion. The surface membrane and the nuclear membrane remain apparently unaffected until the cytoplasmic disruption is very advanced. There is no apparent adverse effect on the structure of host lymphocytes, including mitochondria, which emphasizes the parasite-specificity and narrow spectrum of the mode of action of Buparvaquone.