Анотация: To determine the effect of sheep breed upon blood serum lysozyme and complement activities, 432 blood samples were collected from the following sheep breeds: Mouton Charollais, Ile de France, Trakia Merino, milk crosses, White Maritsa, Patch-faced Maritsa, Karnobat local, Pleven Blackhead and Stara Zagora (48 sheep from each breed). The animals were reared at the Agricultural Institute - Stara Zagora, the Agricultural Institute - Karnobat and private farms. The highest alternative pathway of complement activation (APCA) activity was observed in Ile de France sheep (p<0.01), and the lowest in milk crosses reared at the Agricultural Institute - Stara Zagora. Blood lysozyme concentrations exhibited the opposite trend: highest values in milk crosses (p<0.01), and lowest in White Maritsa and Ile de France sheep