A trace amount of chlorophyll (Chl) a was found in addition to protochlorophyllide (PChlide) a and protochlorophylls (PChls) a in acetone extracts of etiolated cucumber cotyledons by HPLC. There was, however, no chlorophyllide (Chlide) a, which was present in extracts from illuminated cotyledons. After 10 min exposure of cotyledons to light, all PChlide a was photoreduced to Chlide a. Hence, the whole pool of PChlide seems to be photoactive. The "photoinactive" spectral form PChl(ide) F633 was identified as a mixture of monovinyl-PChls a esterified by different alcohols.