We described recently a new Chl a b binding polypeptide in PS II from spinach (Irrgang, K.-D., Bechtel, C., Vater, J. and Renger, G. (1990) in Current Research in Photosynthesis (Baltscheffsky, M., ed.), Vol. I, pp. 375–378, Kluwer, Dordrecht). This 14–15 kDa chlorophyll a b binding polypeptide complex was isolated and purified from intrinsic photosystem II membrane polypeptides. About 3 Chl a and 1 Chl b were bound per protein molecule. A polyclonal antiserum was induced against this pigment-protein complex. SDS polyacrylamide gradient gel electrophoresis in combination with immunoblotting revealed reactivities with two polypeptides of very similar relative molecular masses of 14 and 15 kDa that are clearly identified in thylakoids, PS II membrane fragments, ISO and RSO thylakoids. Their N-termini were blocked by an as yet unidentified modifying group. Using analytical isoelectric focusing under denaturing conditions their isoelectric points were determined to be 5.2–5.3 and around 6.0–6.5. The isolated polypeptides of the pigment-protein complex tend to self-associate into oligomeric forms of about 66–70 kDa. Furthermore, under mild solubilisation conditions an oligomeric pigment-protein complex of 120 kDa was observed. This oligomer was shown to be heterogeneously composed of the 14–15 kDa proteins and at least another pigment-binding polypeptide with an M r of 22–24 kDa. The low molecular mass pigment-protein complex ( CP14–15) is proposed to act as an additional antenna complex within PS II.