AbstractTwo dimensional thin‐layer chromatography on anion‐exchange cellulose enables the separation of the normally occurring ribo‐ and deoxyribonucleoside triphosphates. This technique was applied to perchloric acid extracts of callus tissue of sycamore and tobacco and of pine pollen grown in32P‐orthophosphate labelled media to quantitate the nucleoside triphosphate pools under different growth conditions.The results showed that the ratio of the deoxyribonucleo‐side triphosphates to their corresponding ribonucleoside triphosphates is low in plant cells, similar to the ratio previously found for animal cells.During the period of most rapid DNA synthesis in the callus tissue, the deoxyribonucleoside triphosphate pools reach their highest values. This effect is also demonstrated with cells ofEscberi