Hypothymic BALB/c.nu/nu mice are more resistant than intact BALB/c.nu/ + mice to Babesia rodhaini and a proportion survive a dose of infected blood which is uniformly lethal to nu/ + mice. This proportion of nu/nu survivors is not affected by administration of an anti-Babesial drug, whereas the majority of nu/ + mice develop a long-lasting resistance to infection. The data suggest that T cell dependent activities are involved both in the acceleration of parasitaemia and in the development of drug-assisted resistance.