Bean leaves exposed to cycles of 2 min light-118 min dark, (1) can photoreduce cytochrome f; (2) have cytochrome b-559HP; (3) have a DCMU (3-[3′,4′-dichlorophenyl]-1,1-dimethylurea)-sensitive Hill reaction. Detergent extracts of the plastids show only pigment-protein complex I. Shortening the dark period leads to formation of chl b (chlorophyll b) and the appearance of pigment-protein complex II with no increase in the rate of Hill reaction. As the proteins of detergent solubilised etioplasts and of plastids from leaves flashed for 60 h are very similar, it is concluded that pigment is added on to the pre-existing proteins of the etioplast. The presence of pigment-protein complex II reflects the requirement for chl b to stabilise it during extraction rather than the activity of PS II (Photosystem II).