In tissue cultures of Nicotiana tabacum L. ribulose-diphosphate carboxylase and NADP-dependent glyceraldehyde-3-phosphate dehydrogenase can be found only after transfer of the tissue to light. The increase of the activities of these enzymes parallels that of chlorophyll.The increase in ribulose-diphosphate carboxylase is not associated with a decrease in glucose-6-phosphate dehydrogenase activity as described for rye seedlings (Feierabend, Planta 71, 326-355, 1966). If chloramphenicol is added to the cultures before greening no ribulose diphosphate carboxylase activity can be found, whereas there is no effect on the activity of glucose-6-phosphate dehydrogenase. It is shown that glucose-6-phosphate dehydrogenase activity changes with the glucose concentration of the growth medium.