Sera from 60 horses held in breeding herd in Brazil were examined monthly by ELISA, immunofluorescence antibody test (IFAT) and Western blot. All foals had maternal antibodies detectable by ELISA and IFAT, and sero-conversion took place between the 2nd and 5th month of age. The 48 and 50 kDa antigens were recognized first in the course of infection. Of 79 sera taken after sero-conversion 78 reacted with the 48 kDa antigen, 76 with the 50 kDa, 50 with the 70 kDa, 54 with the 112 kDa, 72 with the 141 kDa antigen. In general, sera from horses older than 1 year reacted with all 5 diagnostic antigen bands, but sera from horses older than 3 years only weakly. The antigens of 48 and 50 kDa appear to be conserved among all strains of B. caballi examined so far and are consistently recognized by all infected horses. They are the target antigens for a serological test based on antigens produced by recombinant DNA techniques.