A 5-year-old neutered male Mastiff and an 8-year-old spayed female Labrador Retriever were presented to the University of Minnesota Veterinary Medical Center. The Mastiff was presented for evaluation of lameness and pyoderma one month prior in Missouri, where he tested positive for Ehrlichia canis by serum ELISA test, treated with doxycycline. PCR for Ehrlichia sp, Anaplasma sp, Babesia sp, and Bartonella sp, and PCR for antigen receptor rearrangement were negative, serum protein electrophoresis (SPE) revealed polyclonal gammopathy, and mildly reactive lymphoid cells were seen cytologically. The Labrador presented with a proliferative rostral mandibular gingival mass and lipomas for further presurgical evaluation of cold agglutinin activity documented by a commercial laboratory 2 years earlier prior to removal of a grade II mast cell tumor. This dog had a negative SNAP4Dx, normal SPE, and persistently increased serum ALP activity and polyuria/polydipsia suggestive for hyperadrenocorticism. Both dogs had markedly agglutinated RBC in the EDTA samples that dispersed with warming, and normal plasma color. Cold agglutinin activity was demonstrated by direct saline agglutination testing using whole blood and washed erythrocytes demonstrating agglutination at 30°C, 25°C, 15°C, and 4°C, but not at 37°C. CBC results (ADVIA 2120i) from the Mastiff revealed no significant differences in the RBC results obtained at room temperature (RT) and at 37°C; however, the RT run demonstrated negative bias in neutrophil and platelet concentrations attributed to rapid RBC settling. This uncommon hematologic condition may cause artifacts on the automated leukogram and platelet count, and may be subclinical for long periods.