Nondialyzable substances similar to blood group A antigens occur in White Leghorn embryos, and in partially purified egg-grown influenza virus preparations. The presence of material with group A activity in the virus itself is not proved. Commercial influenza virus vaccines which were tested contained non-dialyzable substances with group A activity. Injections of this nondialyzable material into human volunteers caused an increase in anti-human blood group A(1) and A(2) antibodies.