There is a conventional method for the determination of hemagglutination-inhibiting (HI) antibody titer of human serum against influenza virus. In this method it is necessary to add such solutions as receptor destroying enzyme (RDE) and KM, to serum to remove an inhibitor from the serum. Moreover, antigen solutions and chicken erythrocyte suspension have naturally to be added.Accordingly, it is impossible to determine an HI antibody titer lower than 1: 16 by this method. In the present study, it was attempted to develop a new method sensitive enough to detect such a low titer. In it, a strain of influenza A type virus was sensitized with normal guinea-pig serum to prepare a strain resistant to an inhibitor. This and the conventional strains were compared with each other in the determination of the titer of HI antibody in the serum which had been treated with RDE and to which bouillon had been added.The results obtained are as follows.<BR>1) The A/Tokyo/75 type and the A/Victoria/75 type strain of influenza virus were treated with normal guinea-pig serum (heated at 60°C for 30 minutes). A strain resistant to the inhibitor was obtained after several generations had passed.<BR>2) There were no large differences in such biological characters as infectivity to eggs and thermostability between the conventional strain and the resistant one mentioned above.<BR>3) With the conventional and the resistant strain as antigens, the conventional method was employed to determine the HI antibody titer of RDE-treated serum, and essentially the same titer was obtained, regardless of the strain used as antigen.<BR>4) With the resistant strain as antigen, HI antibody titer was determined in RDE-treated serum and bouillon-added serum after inactivation of each serum at 56°C for 30 minutes. There was, however, no large difference in the titer determined between the two sera.<BR>5) When a portion of a serum sample was treated with RDE and subjected to the conventional method, it showed an HI antibody titer of less than 1: 16. Another portion of the serum sample was not treated with RDE, but was diluted directly immediately after inactivation at 56°C for 30 minutes.When HI antibody titer was determined in the resulting dilutions with the resistant strain as antigen, titers lower than 1: 4 were presented by 54 and 51 per cent of the human sera infected with the A/Tokyo/75 type and A/Victoria/75 type strain, respectively.<BR>6) Essentially the same results were obtained even from a resistant strain prepared by concentration of the guinea-pig serum fraction.