Two novel sub-thylakoid fractions from Pisum sativum L. containing the photosystem I reaction centre and peripheral light-harvesting complex were characterised. Both preparations displayed a number of spectral and biochemical properties in common with those exhibited by a standard photosystem I preparation. However, both differed markedly in their sedimentation properties when compared with the standard preparation. In addition, one of the novel preparations was found to contain large amounts of the chlorophyll a/b-binding light-harvesting complex of photosystem II (LHC II). It was shown that excitation energy is transferred from the LHC II component to the photosystem I component within this preparation. Such excitation energy transfer has not previously been demonstrated in a sub- thylakoid preparation. Monoclonal and polyclonal antibodies were raised to the apoproteins of the chlorophyll a/b-binding peripheral light-harvesting complex of photosystem I (LHC I). Assays of polyclonal antibody specificity showed immunological cross-reaction between individual LHC I apoproteins and between the apoproteins of the LHC I and LHC II. The immunological cross-reactivity between the polypeptides of the LHC I and LHC II were shown to be spurious. The LHC I apoproteins were found to fall into two distinct immunological groupings which were supported by assays of monoclonal antibody specificity and by a comparison of their partial peptide maps. Immunoblot assays showed that the LHC I apoproteins failed to accumulate in the thylakoid membrane under intern intermittent illumination. When greening pea seedlings were placed in the dark, the LHC I apoproteins not only failed to accumulate but underwent a net loss during the dark period. In vivo radiolabelling experiments showed that the apoproteins of the LHC I continued to be synthesised even after 16 h darkness. The failure of these proteins to accumulate in the thylakoid in the dark is therefore due to protein turnover.