Pseudomonas aeruginosa alginate was purified and characterized in terms of uronic acid, carbohydrate and protein content, as well as by infra-red spectroscopy and gel electrophoresis. Added exogenous bacterial alginate inhibited the uptake and degradation of both viable and non-viable radiolabelled non-mucoid P. aeruginosa by resident mouse peritoneal macrophages. Alginic acid (from seaweed) inhibited the same parameters to almost the same degree. Bacterial alginate also inhibited the uptake of fluorescent-labelled zymosan and latex particles. Starch, at equivalent viscosity to the alginate, inhibited the uptake and degradation of radiolabelled nonviable P. aeruginosa to a greater extent, but Dextran T500 had no effect. This suggests that the viscous nature of alginate exerts a non-specific inhibitory effect on the uptake and subsequent degradation of phagocytosible particles.