A dimer of chlorophyll-protein Complex II (CP II) can be separated in high yield by electrophoresis in sodium dodecylsulphate (SDS) polyacrylamide gels after solubilisation of chloroplast thylakoids in 1% Triton X-100 or Triton/S SDS mixtures. Its half life at 4°C is approx. 6 days. The dimer is readily converted to the monomer by addition of SDS to thylakoid extracts after solubilisation in Triton. Conversion of monomer to dimer by Triton addition to SDS solubilised extracts was not achieved.