When the incubation period of primary isolation plates was extended to 48 h, mucoid strains of Pseudomonas aeruginosa were found in specimens from various infected sites in patients who did not have cystic fibrosis. The 17 mucoid isolates were characterised in terms of mucoid type, pyocin type, and their sensitivity or resistance to seven beta-lactam and two aminoglycoside antibiotics. The carbohydrate, uronic acid (alginate) and protein content of the water-soluble extracellular material of 15 strains was determined. This material was fractionated by ion-exchange chromatography, and the presence of alginate confirmed by the chemical assay of uronic acids and their quantitation by gas-liquid chromatography. Uronic acids were absent from a non-mucoid revertant of one strain. The strains produced alginate with a high content of mannuronic acid and substituted with O-acetyl groups. By proton nuclear magnetic resonance (1H-nmr) analysis the alginate from three strains was shown to lack polyguluronate blocks in its structure. These properties are also found in the alginate of mucoid P. aeruginosa strains from patients with cystic fibrosis.