SummaryThe immunoprophylactic capacity of colostrum‐derivedRhodococcus equiantibodies was evaluated in pony foals experimentally infected withR. equi.Six pony mares (Group 1) were injected serially with liveR. equiduring pregnancy; a further six pony mares were not vaccinated and used as controls All (Group 2) foals were permitted to suck from their dams and the adequacy of IgG, passively transferred via colostrum, was determined by single radial immunodiffusion. The presence and relative value of R. equi specific antibodies were determined by ELISA. Mares vaccinated withR. equideveloped significantly (P<0.05) increasedR. equiantibody levels, and the principal foals from these mares had specific antibody values that were significantly (P<0.01) higher than control foals.Rhodococcus equiwere aerosolised into a caudal lung lobe of all foals at seven days of age. Clinical signs, haematological alterations, immune responses and thoracic radiographs were monitored. Foals were subjected to euthanasia and complete post mortem examinations were performed. All foals developed pneumonia as evidenced by clinical and radiographic signs. The survival rate for Group 1 foals did not differ significantly from that of control foals. Two of the six foals in each group survived the bacterial challenge. Also, there were no significant differences between groups for duration of survival, peak febrile responses, duration of fever, time from infection to onset of fever, or peak concentrations of leucocytes and neutrophils. Post mortem examinations of four foals from each group, destroyed approximately three weeks post infection because of terminal disease, revealed severe pyogranulomatous pneumonia involving primarily the caudal portion of one lung. One control foal had miliary pyogranulomatous lesions in both lungs.Rhodococcus equiwas cultured from the pulmonary lesions in these foals. Two principal and two control foals that recovered from the infections had mild pulmonary fibrosis at three months post infection.Rhodococcus equiwas not cultured from any of these foals at necropsy. The results of this investigation suggest that passive immunisation of newborn foals, by ingestion of colostrum derivedR. equispecific antibodies, does not provide adequate protection against experimental infection withR. equ