Two Pseudomonas aeruginosa alginates were lysed by an overexpressed polymannuronate lyase AlxMB (only acting on two or more consecutive, nonacetylated mannuronate units) to prepare either mannuronate blocks (poly-M blocks) with dp approximately 30, or strictly alternating sequences of mannuronic and guluronic acid (poly-MG blocks) with dp > 20. The poly-M blocks were obtained by lysis of a P. aeruginosa polymannuronate that has 50% O-acetylation at C-2 and C-3. The poly-MG blocks were obtained from a P. aeruginosa alginate that contained both mannuronate and guluronate residues. The polysaccharide was first deacetylated and then treated with the lyase to excise the mannuronate units from the alternating-MG blocks. Both types of blocks should have potent biological effects and should provide useful specific substrates for characterisation of other alginate lyases.