Cotyledons of cucumber plants grown in the dark for 7 days were treated with various concentrations of cytokinins for 14 h and then moved into light. After 3 h the treated cotyledons had up to 450% more chlorophyll than the water controls. This suggests that cytokinins have an important role in the formation of chlorophyll. The increase in chlorophyll level was proportional to cytokinin concentration and was apparent at concentrations as low as 0.001 mg/l. Sensitivity to cytokinins depended on the age of the cotyledons and the time of exposure to light. Gibberellic acid, indoleacetic acid, adenine and sucrose did not cause a similar increase in chlorophyll levels. This effect of cytokinins on chlorophyll formation is valuable as a simple, rapid bioassay for cytokinins.