The ability of various oral streptococci to bind salivary alpha-amylase to their cell surfaces was investigated. Samples of cells were mixed with whole cleared saliva and the alpha-amylase remaining after removal of the cells was assayed by radial diffusion in starch-containing agarose. Seventy-five per cent of Streptococcus sanguis strains bound the enzyme but strains of Streptococcus mutans and Streptococcus mitior did not. SDS-polyacrylamide gel electrophoretic analysis of Strep. sanguis cells which had been mixed with saliva showed that alpha-amylase is bound to the surface of the cells and can be recovered from them intact. The ability of Strep. sanguis strains to bind the enzyme did not correlate with biotype, IPS production or ability to adhere to saliva-coated hydroxyapatite. The cell-surface receptor responsible for the binding of alpha-amylase to Strep. sanguis (NCTC 7865) is a heat-stable component, possibly protein in nature, which is closely associated with the cell wall.