Sucrose (but not other sugars) suppresses chlorophyll synthesis in a carrot-callus strain grown on a medium on which free-space invertase does not develop (Heller's mineral elements, thiamine 0.1 mg/1, IAA 0.01 mg/1, sucrose 3 per cent). This suppression was not caused by trace contaminants in sucrose or by the lack of available iron in the medium. Suppression of chlorophyll synthesis by sucrose was reversed by transfer of growing tissue to a medium on which high free-space invertase activity developed (Murashige and Skoog mineral elements, White's vitamins, 2, 4-D 0.05 mg/1, sucrose 3 per cent). Feeding of porphyrin precursors, and tests of Krebs cycle function, were employed in an attempt to locate the sucrose-inhibited step in chloroplast biogenesis.