A crude homogenate obtained from greening cucumber (Cucumis sativus, L.) cotyledons in tris-sucrose, pH 7.7, containing coenzyme A, glutathione, potassium phosphate, pH 7.7, methyl alcohol, magnesium, nicotinamide adenine dinucleotide, and adenosine triphosphate, is able to incorporate 4-(14)C-delta-aminolevulinic acid into chlorophyll a and b in the presence of oxygen. If the homogenates are prepared from etiolated cotyledons which have been exposed to light for two and one-half hours, (14)C-chlorophyll a is synthesized. However, when the homogenates are prepared from cotyledons illuminated for four and one-half hours, both (14)C-chlorophyll a and b are produced.