The growth of green cultures of callus tissue from Nicotiana tabacum var. "Samsun" is stimulated by light. To determine whether the increase in growth is caused by photosynthesis or by a blue light dependent increase of protein synthesis, a comparative study was made of the effect which blue and red light have on the growth and the composition of tobacco tissue. It is shown that the growth stimulation by light depends on the chlorophyll content of the tissues. Starting with chlorophyll-free tissue the cultures begin to grow faster in blue light only after they become visibly green. On the other hand, the growth of green tissue in red light decreases as soon as the chlorophyll content under this condition becomes less. There are no differences in the rate of growth of green tissues cultivated in blue and in red light of approximately the same flow of quanta; in both cases the cultures grow better than the controls in the dark. Furthermore there are no differences between the protein and carbohydrate content of tissues grown in blue or red light and in the dark. There is, however, a small but significant difference between the total nitrogen of green tissue and that of chlorophyll-free tissue which is due to a higher amount of soluble nitrogen in the green tissue. From these results it is concluded that the light dependent growth stimulation is caused by photosynthesis. As shown by a light dependent (14)CO2 incorporation in which sucrose is the main product, the green cells are able to fix CO2 photosynthetically. However, the rate of photosynthesis in the tissue cultures is small and does not balance the respiration. It seems very unlikely, therefore, that the formation of carbohydrates by photosynthesis is responsible for the observed growth increase.