Photosystem II core particles were isolated, using the method of Gounaris and Barber (Gounaris, K. and Barber, J. (1985) FEBS Lett. 188, 68–72) from [ 32P]phosphate- and [ 35S]methionine-labelled thylakoids of pea ( Pisum sativum) chloroplasts. SDS-polyacrylamide gel analysis of the PS II core particles showed diffuse Coomassie blue stained polypeptide bands centred at molecular mass of 43, 40, 33, 30 and 10 kDa. Subsequent fluorography of the SDS-polyacrylamide gels revealed 32P-containing bands at 55, 43, 40, 33 and 30 kDa and a 35S-containing band, representing the D1 (herbicide-binding) protein, at slightly higher than 33 kDa. The 30 kDa [ 32P]polypeptide which was the major 32P-labelled species was therefore assigned to be the D2 protein, a conclusion reinforced by its degradation by the lysine-specific endoprotease, Lys-C. The approx. 33 kDa D1 protein, however, did not appear to be phosphorylated, since the weaker 32P-containing 33 kDa band was degraded by the lysine specific enzyme, whilst the [ 35S]D1 band was unaffected. Estimation of cytochrome b-559 in different fractions of the sucrose density gradient used to prepare PS II core particles indicated that the cytochrome was confined to the PS II core fraction, whereas the 10 kDa 32P-labelled component was present in other fractions. Moreover, the 10 kDa 32P-labelled polypeptide was readily degraded by the lysine-specific endoprotease, a result not expected for the lysine free cytochrome b-559 10 kDa polypeptide. It is, therefore, also concluded that the 10 kDa [ 32P]component cannot represent the cytochrome b-559 apoprotein.