Adherence through carbohydrate-binding adhesins is an early step in colonization of the lung by gram-negative organisms, and because published data indicate that binding involves mannose groups, we tested the ability of a β-linked acetylmannan (acemannan) to inhibit adherence of Pseudomonus aeruginosa to cultures of human lung epithelial cells. Adherence of radiolabelled P.aeruginosa to A549 cells (a type II-like pneurnocyte line) increased linearly with the duration of the incubation. Acemannan inhibited adherence of bacteria, and the extent of inhibition was related to the concentration of the mannan. Inhibition required continued contact between acemannan and the target epithelial cells; cells washed free of acemannan no longer discouraged bacterial binding. Comparison of binding between seven strains of P.aeruginosa indicated that fewer mucoid than non-mucoid bacteria adhered, but binding of either phenotype was inhibited by acemannan. Mannose methyl α-D-mannopyranoside, methyl β-D-mannopyrannoside and dextran did not affect adherence of any of the non-mucoid strains. Mannose inhibited adherence by one mucoid strain, but not the other, indicating differences between strains of the same phenotype. Since prior treatment of epithelial cells with concanavalin A did not affect acemannan-induced inhibition of bacterial adherence, we concluded that the inhibitory effect of acemannan probably does not involve mannose-containing receptors.