Hemagglutinin ofFusobacterium nucleatumwas extracted from Triton X‐100‐pronase P‐treated cell envelopes, and was purified by affinity chromatography on L‐arginine agarose. The hemagglutinin was inactivated by heating at 70°C for 1 min. The activity was inhibited by L‐arginine but was not affected by any sugars or by EDTA. The hemagglutinin aggreggated 14 out of 17 strains of oral streptococci tested, and the bacterial aggregating activity was also inhibited by L‐arginine. The results indicate the dominant role of this hemagglutinin in the adherence of this bacterium both to host cells and to ot