THE possible use of nonspecific immunity to control Babesia infections has not been much examined. Cattle treated with Escherichia coli lipopolysaccharide (1.25 µg per kg live weight) immediately before infection with B. argentina ( =B. bovis) were slightly more resistant than controls1. An interferon inducer, a double stranded RNA, slightly delayed death in mice infected with B. rodhaini2. The question has been reopened by some work with B. microti and B. rodhaini in which mice injected intravenously (i.v.) with 2×107 Mycobacterium bovis strain BCG (Glaxo strain) became completely immune to intraperitoneal challenge with 106 Babesia parasites3. These remarkable results suggested that the method should be examined in domestic animals. We decided to assess it in calves and report here that BCG fails to protect against infection with B. divergens, the common cause of redwater in the UK.