Lectins of various specificities were examined for interaction with strains of oral streptococci of various species. The lectins were capable of binding galactose, Nacetylgalactosamine, glucose, N-acetylglucosamine, mannose, fucose and sialic acid. Lectin reactivities were strain-dependent in that some members of a species, but not others, could be aggregated by certain lectins. Proteolysis and extraction with hot water, guanidine hydrochloride and sodium dodecyl sulphate tended to increase the reactivity of the streptococci with lectins but did not change the recognition patterns of the bacteria. Sonication, in contrast, tended to reduce the ability of streptococci to be agglutinated by lectins. Furthermore, lectin reactivities were dependent on the growth conditions, as evidenced by changes in lectin titres following streptococcal growth in subinhibitory concentrations of citrate, fluoride or antibiotics. It is likely that lectins could be useful tools for epidemiological studies and to probe strain-dependent and growthdependent surface characteristics of viridans streptococci.