An immunoprophylaxis program for R. equi infection of foals has been established on a number of thoroughbred breeding farms in Argentina over the past 4 years. Nearly 800 mares annually were immunized subcutaneously during the last 2 months of pregnancy with 2-3 doses of a vaccine containing soluble antigens of R. equi, including the virulence associated protein (VapA) and 'equi factors' exoenzymes. The mortality from R. equi pneumonia in the foals from vaccinated dams dropped from an average of 3% in the 5 years before the vaccination program was initiated to an average of 1.2% in the 4 years during which the program was applied (P < 0.02). On 3 farms, an additional 380 foals of vaccinated dams annually over 3 years also received at 25 days of age 600-1200 ml of hyperimmune plasma from donors immunized with this vaccine, and as well at 4 days of age in foals with poor transfer of R. equi antibodies from their dams. The average foal mortality because of R. equi in the 380 foals annually to which hyperimmune plasma was administered dropped from 5.8% on these 3 farms to 0.2% (P < 0.05). Active vaccination of foals of unvaccinated mares on an enzootic farm at 20, 30, and 40 days of age did not protect them from mortality due to R. equi pneumonia. Serology was done by complement fixation and an agar gel immunodiffusion (AGID) tests using antigens prepared in the same manner as the vaccine antigens. The immune responses among hyperimmune plasma donors varied considerably as did the responses of vaccinated mares. Of 1117 serum samples with normal post suckling gammaglobulin levels (> 600 mg%) collected at 2 days of age from foals of vaccinated mares, 36% showed a negative or weak positive AGID reaction, while the remainder had positive to strongly positive reactions.