In alkaline aqueous solution, the sedimentation coefficients of DeoxyHb, HbO(2) and HbCO of Lampetra fluviatilis L. are 1.9 +/- 0.1 S. This value corresponds to a molecular weight of about 17,000, i.e. the value of a single haem polypeptide chain. In weak acidic conditions, both the non-liganded DeoxyHb and the liganded forms, HbO2 and HbCO, associate to form dimers and oligomers. The monomer-oligomer transitions of these compounds take place at different pH values: DeoxyHb pHO.5 approximately 6.7; HbO(2) and HbCO pH(O.5) approximately 5.9. With respect to the association modus, the equilibrium: 4 Hb &rlhar2; 2 Hb(2)&rlhar2;Hb(4) may be preferred.