A method of treating Theileriosis in cattle and sheep both therapeutically and prophylactically, comprising the administration of an effective amount of a compound which is a 2-hydroxy-1,4-naphthoquinone derivative of formula (I) or a pharmaceutically acceptable salt thereof, ##STR1## WHEREIN N IS ZERO OR 1 TO 12, AS ACTIVE INGREDIENT, TO AN ANIMAL WHICH IS SUSCEPTIBLE AND INFECTED WITH OR POTENTIALLY EXPOSED TO AN INFECTION WITH Theileria species.
