The interaction of five anti-pseudomonas antibiotics with both commercial and pseudomonas alginates was studied by investigation of their binding and diffusion characteristics. The two sources of alginate were qualitatively but not quantitatively similar in these respects. Unlike the beta-lactams, gentamicin and tobramycin bound avidly to both sources of alginate and, when the alginate gel to antibiotic ratio was high, the aminoglycosides exhibited diffusion coefficients which were approximately 20% of the beta-lactam values. At much lower ratios of alginate to antibiotic the aminoglycosides caused precipitation in the alginate with apparent disruption of the gel structure, and ultimately penetrated the gel at a faster rate than the beta-lactams. The strong aminoglycoside binding to alginate was reduced, but not eliminated by the presence of physiological concentrations of salts.