Summary Fowl, sheep, rabbit, guinea-pig and human erythrocytes were agglutinated by cells of Leptotrichia buccalis ATCC 14201 and 19616. Agglutinating activity was destroyed by heat treatment (80°C, 10 min) or incubation with alcohol (95 per cent, 45°C, 30 min). Trypsin treatment did not reduce activity. In hapten-inhibition assays with simple sugars, lactose and N-acetyl-D-galactosamine inhibited the agglutinating activity of the bacteria for human erythrocytes. When bacterial cell suspensions were stored at 4°C for 1 week, sensitivity of the cells to these inhibitory sugars was increased. The data suggest that the receptor sites on human erythrocytes for Lept. buccalis may be sugar in nature. Bacterial haemagglutination was also inhibited by 5 mM EDTA. Electron microscopy revealed a nap of short hair-like projections (30 nm in length and 3 nm in diameter) on the outer surface of both strains. Both strains of Lept. buccalis exhibited enhanced adherence to saliva-coated enamel powder. Adherence was markedly impaired by sugars which exhibited an inhibitory effect on haemagglutinating activity. Both strains were agglutinated by whole saliva supernatant, and the saliva-induced aggregation was inhibited by those sugars which inhibited both haemagglutination and adherence to saliva-coated enamel powder. The data suggest that the salivary aggregating component may possess the same specific group with which Lept. buccalis reacts with on human erythrocytes, and that the enhanced adherence of Lept. buccalis cells to saliva-coated enamel powder is due to a greater affinity of the organism for this component which adsorbs to human enamel powder.