Babesia bovis merozoite apical membrane polypeptide Bv60 was found to be rhoptry associated by immuno-electron microscopy and was redesignated rhoptry-associated protein 1 (RAP-1). The N-terminal 300 amino acids of RAP-1 have a high level of sequence similarity to the same N-terminal region of p58, its homolog from Babesia bigemina. However, the interspecies conserved sequences did not include RAP-1 surface-exposed B-cell epitopes as defined by monoclonal antibodies. Furthermore, neither heterologous B. bigemina immune nor monospecific anti-p58 bovine serum binds to whole RAP-1, indicating that the major B-cell epitopes recognized by these sera are also not encoded by the conserved sequences. Truncated RAP-1, expressed by a subclone encoding the N-terminal 235 amino acids, is weakly bound by antibodies in heterologous sera. A peptide representing the longest conserved amino acid sequence (amino acids 121 to 134) in this region is also weakly bound by antibodies in immune bovine sera, and rabbit antibodies induced by and strongly reactive with the peptide alone fail to bind native or denatured RAP-1. Thus, although the conserved region may contain one or more poorly immunogenic B-cell epitopes, these epitopes are inaccessible to antibody in whole RAP-1. The results indicate that the major immunogenic B-cell epitopes of RAP-1, including surface-accessible epitopes bound by monoclonal antibodies, are distinct from the conserved sequences representing putative functional domains.