Abstract The monomer molecular weight of phycocyanin was determined by sedimentation equilibrium measurements in 6 mol/1 guanidine hydrochloride (GndCl) and by sodium dodecyl sulfate (SDS)–polyacrylamide gel electrophoresis. The former measurement gave the monomer molecular weight of 4.0×104. From the latter, the phycocyanin was found to be composed of two kinds of subunits, the molecular weights of which were 1.7×104 and 2.0×104. Assumimg that the phycocyanin monomer consists of two polypeptide chains, the molecular weight becomes 3.7×104. These values coincide with the value of 4.2×104 assumed in our previous works, which corresponds to onesixth of the hexamer molecular weight. The sedimentation coefficient, s25w, was 2.21 Svedberg unit, and the intrinsic viscosity in 6 mol/1 GndCl was 19.5 ml/g, at 25.0 °C. From these values, it was clear that the phycocyanin monomer was unfolded in the solvent.