Abstract A commercial λ-carrageenan preparation was dissolved in 3·0 m ammonium sulphate and loaded onto a Sepharose CL-4B column equilibrated with the same solvent. Fractionation was carried out by stepwise elution with decreasing concentrations of ammonium sulphate at a low temperature. Each fraction obtained was analysed for sulphur content, 3,6-anhydro- d-galactose content, and molecular weight (Mw). The Mw of all the fractions was lower than the Mw of the starting material. The species with the highest sulphur content and lowest 3,6-anhydro- d-galactose content, a composition closest to that of an ideal structure, was recovered in the 1·5 m fraction.