When two types of alginate lyases [poly-mannuronate (poly M)-specific and poly-guluronate (poly G)-specific lyases] were incubated on agar plates containing alginate and the surfaces of the plates covered with CaCl2 solution, poly M-specific and poly G-specific lyases yielded white-halos and white-rings, respectively. The substrate specificity of alginate lyase was easily determined by discriminating between the types of gellation (i.e. halo or ring formation) caused by the interaction between calcium ions and depolymerized alginates.