A blood sample received from a field case of redwater in Eire caused simultaneous infections of Babesia divergens and Ehrlichia phagocytophila when inoculated into a splenectomised calf. Each disease agent was isolated in a separate splenectomised calf by suppressive drug treatment of the other agent, and blood stabilates were made. The disease agents were inoculated synchronously or asychronously into groups of calves, whose reactions were observed. Infection with B divergens had no effect on the severity of reaction of the calves to E phagocytophila but E phagocytophila infection tended to delay the establishment of B divergens if inoculated simultaneously or one week previously. If the inoculations were simultaneous there was also a tendency for the B divergens infection to be partially or completely suppressed. There was no evidence of recrudescence of one infective agent when the other was inoculated one month later.