Diagnostic tools for serodiagnosing ehrlichiosis in mammals, particularly in members of the Canidae family and in humans are provided. The diagnostic tools are a group of outer membrane proteins of E. chaffeensis and variants thereof, referred to hereinafter as the "OMP proteins", a group of outer membrane proteins of E. canis and variants thereof referred to hereinafter as the "P30F proteins", and antibodies to the ONP proteins and the P30F proteins. The OMP proteins of E. chaffeensis encompass OMP-1, OMP-1A, OMP1-B, OMP-1C, OMP1-D, OMP1-E, OMP1-F, OMP1-H, OMP-1R, OMP-1S, OMP-1T, OMP-1U, OMP-1V, OMP-1W, OMP-1X, OMP-1Y and OMP-1Z. The P30F proteins of E. canis encompass P30, P30a, P30-1, P30-2, P30-3; P30-4, P30-5, P30-6, P30-7, P30-8, P30-9, P30-10, P30-11, and P30-12. Isolated polynucleotides that encode the E. chaffeensis OMP proteins and isolated polynucleotides that encode the E. canis P30F protein are also provided. The present invention also relates to kits containing reagents for diagnosing human ehrlichiosis and canine ehrlichiosis, and to immunogenic compositions containing one or more OMP proteins or P30F proteins.