Spikeless particles of HVJ (Sendai virus) lacking in hemagglutinating (HA) activity were obtained by enzymatic digestion of virions with trypsin followed by centrifugation through a sucrose gradient. When they were mixed with glycoprotein components of Newcastle disease virus (NDV) obtained by treatment of purified virions with deoxycholate (DOC), the mixture showed hemagglutination reaction, which was inhibited by anti-NDV serum, but not by anti-HVJ serum. Sedimentation profile of the HA active agents was then examined by centrifugation of the mixture of spikeless particles of HVJ (labeled with 3H-uridine) and glycoproteins of NDV (labeled with 14C-amino acid mixture). The results showed that the peak of HA activity had both of the radioactivities, and that the sedimentation rate of the HA was faster than that of spikeless HVJ but slower than that of intact HVJ. Electron micrographs of such HA active structures showed that they were morphologically closely similar to intact virion of HVJ, although they had neither hemolytic activity nor infectivity. The mixture of spikeless HVJ and glycoproteins of HVJ or NDV which were removed from virions by proteolytic enzymes, on the other hand, did not show any detectable hemagglutinating activity.