The adherence of pseudomonal species was investigated by using a newly developed radiometric dacron fiber microcolumn assay. Pseudomonas aeruginosa, P. stutzeri, and Xanthomonas maltophilia were more adherent (approximately 20%) than P. pseudomallei, P. fluorescens, and P. cepacia (approximately 10%). Mucoid strains of P. aeruginosa were consistently more adherent than nonmucoid strains (30% versus 20%). Alginase was shown to inhibit the adherence of mucoid but not nonmucoid P. aeruginosa. Monoclonal antibodies to alginate were also shown to inhibit the adherence of mucoid but not nonmucoid P. aeruginosa. In addition, antibiotics active against P. aeruginosa were shown to inhibit the adherence of both mucoid and nonmucoid strains. Furthermore, synergism between dyadic combinations of monoclonal antibodies and antibiotic (ciprofloxacin), as well as alginase and antibiotic, was also observed. These results indicate that bacterial alginate has an intrinsic role in the adherence of mucoid P. aeruginosa and may have evolved not only for protection against dehydration in the water and soil ecosystem of this bacterium, but also as a means of attaching to soil substrates in the same ecosystem to enhance survival. They also suggest that synergistic combinations of antibiotics with alginase or monoclonal antibodies to alginate may be of value in the therapy of some pseudomonal infections.