A simple method for the simultaneous separation and purification of peripheral blood mononuclear cells (PBMC) and polymorphonuclear neutrophil cells (PMNC) was developed for comparative and functional studies in the immune system of the dog. Purity and cell viability were > 95%, yields were similar to those obtained by other techniques but without red blood cell contamination. Differential blood cell count studies of the isolated cells in blood samples of beagle dogs and German shepherd dogs demonstrated that the 1.077/1.119 double density centrifugation is an effective method of acquiring both highly purified blood mononuclear cells and polymorphonuclear cells as separate entities from the same sample. The interface between plasma and 1.077 contained an average 97% blood mononuclear cells vs. 3% polymorphonuclear cells, and the interface between 1.077 and 1.119 an average 96% polymorphonuclear cells vs. 4% blood mononuclear cells. These data indicate that Histopaque 1.077/1.119 double density gradient allows the purification and physical separation of lymphocytes and phagocytes from a blood sample in the dog, enabling the investigator to examine both cell types from the same sample simultaneously.