The amino-acid sequence of the inhibitory part of the sheep serum inter-alpha-trypsin inhibitor (ITI) was determined. The inhibitor is composed of two covalently linked Kunitz-type domains. The reactive site of the C-terminal antitryptic domain contains arginine in position 71 (P1) and glycine in position 73 (P'2), whereas ITI derived inhibitors hitherto investigated contain phenylalanine in these positions. The reactive site of the N-terminal elastase inhibiting domain contains leucine in position 15 (P1) and methionine in position 17 (P'2), as in ITI-derived inhibitors of pig and horse.