Groups of cattle were infected with Theileria parva by the injection of stabilate material prepared from infected Rhipicephalus appendiculatus ticks. The cattle were treated with the hydroxy alkylated naphthoquinone, menoctone, which was administered intravenously (i/v), intramuscularly (i/m), or orally (p/o) up to 4 days after the disease became apparent. The disease was rapidly controlled and all the treated cattle (15) recovered following (i/v) or i/m injection of a single dose of menoctone at 10 mg/kg body weight but control was incomplete with lower dosages in single or multiple dose regimens. The i/m route was slightly more effective than i/v. The p/o administration of menoctone had only a slight and transient beneficial effect and 10 cattle treated by this route all died of theileriosis. Twelve of 18 untreated controls also died. Following injection of menoctone, schizonts degenerated within the lymphoid cells and piroplasms were destroyed within the erythrocytes. The level and duration of antitheilerial activity in the serum following treatment was assayed by an in vitro culture method and was shown to correlate closely with the efficacy of treatment.