A 2-year field study was performed to determine the capability of increasing Rhodococcus equi specific antibody in foals via plasma transfusion or mare vaccination, to determine the kinetics of R. equi (ELISA) antibody decay and to assess the protective effects of these procedures in foals on a farm endemic for R. equi. Plasma donors were vaccinated with a killed R. equi bacterin and produced high levels of anti-R. equi antibodies, which were harvested by plasmapheresis. In Experiment 1, 68 foals were given 1 litre of hyperimmune plasma intravenously (i.v.) between 1-60 days of age. Foal plasma R. equi antibody was significantly increased and high levels of R. equi antibody (ELISA) were maintained for 60 days. No R. equi pneumonia developed in any foals receiving plasma. In Experiment 2, 99 pregnant mares were vaccinated with R. equi bacterin at 30, 60 and 90 days before foaling. Group 1 foals (101:85 from R. equi immunized mares) also received plasma transfusions and Group 2 foals (14), from R. equi immunized mares, did not receive plasma transfusions. Pregnant mare immunization increased colostrum R. equi antibody significantly. Eight foals showed failure of transfer of specific R. equi antibody. The incidence of R. equi pneumonia was 2.9% in Group 1 foals and 43% in Group 2 foals. Vaccination of pregnant mares did not provide protection against R. equi pneumonia; however, plasma transfusion with hyperimmune plasma administered prior to R. equi exposure was significantly protective in foals.