While studying the hemolytic activity of HVJ, it was hound that various preparations of allantoic fluid virus differed markedly in their hemolytic ability. 0.01M phosphate buffered saline suspension of virus prepared from these allantoic fluids of highly egg-adapted HVJ also showed considerable differences in their hemolytic ability. It was supposed, therefore, that the difference was not due to the different amount of inhibitor contained in each allantoic fluid and also not due to the heritable properties of virus. These findings raised a possibility that there existed some other factors affecting the hemolytic activity of this virus. Indeed, it was observed that the size of inoculum and the incubation period of infected egg markedly affected the hemolytic ability of yielded virus. Hemolytic ability of virus increased with increasing dose of inoculum but decreased as an incubation period was lengthened. Furthermore, it was also discovered that, the hemolytic ability of HVJ decreased during the incubation of allantoic fluid above 12 hours at 37°C and by repeating of absorption to and elution from chick red blood cells.When HVJ transfered from mouse to chick embryo, the variation in hemolytic activity was observed. The mouse passage line virus material prepared from infected mouse lung had no hemolytic activity. On the other hand, the 1st egg-passage virus (from mouse) behaved as a nonhemolytic form but after the treatment of freezing and thawing this virus was found to be highly hemolytic. However, the virus multiplied in the lung of mouse inoculated with 1st egg passag virus was found to have no hemolytic ability. A possibility that this is an example of host controlled variation was discussed.An attempt to find a lecithinase activity in highly hemolytic HVJ particles was made, but no evidence for enzymatic activity in this virus was obtained in this experiment.