The present invention provides a 120-kDa protein gene of Ehrlichia canis, amplified by PCR using primers derived from the DNA sequences flanking the Ehrlichia chaffeensis 120-kDa protein gene. The recombinant E. canis 120-kDa protein contains 14 tandem repeat units with 36 amino acids each. The repeat units are hydrophilic and predicted to be surface-exposed. Also disclosed is that the recombinant E. canis 120-kDa protein is antigenic and reacts with sera from dogs convalescent from canine ehrlichiosis.