The respiration of dog leucocytes suspended in dog serum at rest and during phagocytosis was determined by Warburg manometers. After addition of a suspension of Sarcina lutea, the O2 consumption showed a burst lasting 10-15 min. to values twice resting and remained above the resting level for 11/2-21/2 hrs. Active phagocytosis was demonstrated to have occurred. No respiratory increase attended the addition of India ink, which appeared to injure the leucocytes.