1. 1. Greening barley and pea leaves treated with lincomycin have a reduced chlorophyll content. Lincomycin does not alter the proportion of chlorophyll in chlorophyll-protein complex II (CPII) but greatly reduces that in chlorophyll-protein complex I (CPI). 2. 2. Difference spectra show that chloroplasts from lincomycin-treated leaves are deficient in at least two long wavelength forms of chlorophyll a. These have maxima at 77 K of 683 and 690 nm. 3. 3. The chemically determined P-700/chlorophyll ratio of chloroplasts is unaffected by lincomycin but the photochemical P-700/chlorophyll ratio is less than half of that of the control. It is less affected than the chlorophyll-protein complex I content. 4. 4. Photosystem I activity expressed on a chlorophyll basis is unaffected by lincomycin but the light intensity for half saturation is increased 8-fold. 5. 5. Chlorophyll-protein complex I apoprotein content is reduced by lincomycin. No evidence was found for an accumulation of its precursor(s). The relative abundance of major peptides of 18 000, 15 000 and 12 000 daltons in lincomycin-treated chloroplasts is attributed to a general inhibition of greening and associated membrane formation.