The hemagglutinin-neuraminidase (HN) subunits of NDV and NDV-like isolates were analyzed antigenically by monoclonal antibodies to the HN of Miyadera and Taka viruses. In immuno-double-diffusion (IDD) tests, all NDVs examined gave clear lines of precipitation with some of the potent monoclonal antibodies, but it was difficult to determine with certainty the immunological properties of HN subunits due to a rare disagreement with the results obtained in other immunological tests. Monoclonal antibodies used in the tests were found to show different immunological reactivities with the viruses. Monoclonal antibodies belonging to the 1st group (1/29) inhibited the hemagglutinating (HA) activity of all strains but not the neuraminidase (NA) activity. The second monoclonal antibody (5/205) inhibited both the HA and NA activities of the restrictive NDV strains, indicating antigenic changes in HN molecules. However, the inhibitory activity of this monoclone to neuraminidase appeared to be greatly diminished when neuraminyl lactose was used as substrate. Although the 3rd type of monoclonal antibody (5/220) showed HI activity against several strains, this antibody did not inhibit NA activity of any viruses. The remaining monoclone to the HN of Taka virus inhibited the HA activity of all reference strains of NDV and many NDV-like isolates but did not affect NA activity. Two inhibitory activities of four monoclonal antibodies against different viruses, HI and hemolysis-inhibition, were not always consistent with inhibition of virus growth. HI and NI tests with the above four monoclonal antibodies showed that the strains tested fell into five antigenic groups according to their reaction patterns with mouse hybridoma antibodies.