Radial hemolysis-in-agarose gel (HIG) with members of the genera Aiphavirus and Flavivirus is described. The test can be reproduced with goose and sheep erythrocytes sensitized at the optimal pH for each virus. Human (O group), rooster, guinea pig, mouse and hamster erythrocytes were found to be unsuitable. The titer of immune ascitic fluids in HIG coincides with the titer in hemagglutination inhibition, investigation of convalescent sera after laboratory infection suggests the possibility of HIG application for serologic diagnosis of arbovirus infection.