The present invention is directed to the cloning, sequencing and expression of homologous immunoreactive 28-kDa protein genes, p28-1, -2, -3, -5, -6, -7, -9, from a polymorphic multiple gene family of Ehrlichia canis. Further disclosed is a multigene locus encoding all nine homologous 28-kDa protein genes of Ehrlichia canis. Recombinant Ehrlichia canis 28-kDa proteins react with convalescent phase antiserum from an E. canis-infected dog, and may be useful in the development of vaccines and serodiagnostics that are particularly effective for disease prevention and serodiagnosis.