Coaggregation between Actinomyces viscosus T14V (T14V) and Streptococcus sanguis 34 (Ss34) depends upon specific reaction between lectin on T14V and carbohydrate on Ss34. Studies on coaggregation inhibition by sugars related to D-galactose, beta-galactosides, and amphipathic molecules revealed: (i) D-fucose, D-talose approximately equal to D-galactose, which was 0. 2 potency of lactose. No other hexoses or pentoses inhibited at 0. 1 M. (ii) Gal beta (1 leads to 3)GalNAc alpha OCH2C6H5 was the most potent beta-galactoside inhibitor; it had 20 times the potency of lactose. (iii) Anionic nonaromatic amphipathic compounds were good inhibitors; sodium deoxycholate (I) was equal to lactose; sodium dodecyl sulfate (II) had 15 times the potency of lactose; there was 90 to 100% irreversible inhibition when T14V was treated with 0. 005 M (II). Treatment of Ss34 with II had no effect. (iv) Synergism of inhibition was observed between lactose and I or lactose and II, e. g. , inhibition by 0 VSports手机版. 01 M lactose = 5%; inhibition by 0. 01 M I = 9%; inhibition by 0. 01 M lactose + 0. 01 M I = 87%. (v) The irreversible inhibition by II was prevented when 0. 25 M lactose or 0. 25 M I was present during treatment of T14V with 0. 005 M II. (vi) Synergism and prevention by lactose or by I of irreversible inhibition by II suggest that all three react at the same site on T14V lectin. We hypothesize that the T14V lectin combining site for Ss34 carbohydrate has specific affinity for beta-galactosides and for anionic nonaromatic amphipathic molecules. This site can be saturated by either kind of reagent to exclude the other reagent or to inhibit coaggregation. .