Novel genes encoding homologous immunoreactive thio-disulfide oxidoreductases, or disulfide bond formation (Dsb) proteins from Ehrlichia chaffeensis and Ehrlichia canis are disclosed. While the E. chaffeensis and E. canis Dsb proteins are at most only 31% or less homologous to other known Dsb proteins, the Ehrlichia Dsbs contain a cysteine active site, Cys-Gly-Tyr-Cys, similar to those in known Dsb proteins. As predicted by 15-amino acid identical N-terminal signal peptides, the proteins are primarily localized in the periplasm of E. chaffeensis and E. canis, possibly playing a role in antigenicity and pathogenesis. The present invention provides the nucleotide and amino acid sequences and expression vectors for the E. chaffeensis and E. canis dsb genes, antisera directed against the proteins, and kits to determine whether an individual or animal is infected with a given species of Ehrlichia.