Previous work on hemagglutination in the dog was done with dog''s serum and the results have been conflicting. In this investigation isohemagglutination in normal healthy c? and 2 dogs of various breeds and ages was tested, using serum and plasma. In each expt. groups of 9-12 animals were matched at one time against each other. The Vincent open slide method was employed in most tests. 97 dogs were used and a total of 3355 compatibility tests were done. Breed, sex and age had no influence and iso-agglutination was constant in the individual dog but was definitely affected by distemper. Serum completely freed of fibrinogen did not have any agglutinating power, while fresh plasma showed an incidence of 43% of agglutinations. On standing it gradually lost this power, coincident with the formation of a fibrin precipitate. The phenomenon of agglutination in the dog rested in the fibrinogen fraction of the blood. Dog''s blood hemolysed readily, and hemolysis diminished the agglutination titer. The agglutination titer in dog''s blood was relatively low, as compared to the human. Two definite and one intermediate blood groups were established for the dog.