In order to evaluate whether infection with E. canis alters the expression of major histocompatibility complex (MHC) class I and/or MHC class II receptors, and by doing so alters the immune response to the organism, flow cytometry was performed on DH82 cells infected with Ehrlichia canis (90% infection) and on uninfected DH82 cells of the same passage, using anti-canine MHC class I and II antibodies. MHC class II expression was evident in 47.6 and 46.2% (mean 46.9%) of uninfected DH82 cells using two different anti-MHC class II antibodies, while no MHC class II expression was evident in DH82 cells infected with E. canis. The present results indicate that infection of DH82 macrophages with E. canis down-regulates their MHC class II receptors. These results suggest a possible mechanism by which E. canis evades the immune system.