Bud formation and flowering of Chrysanthemum morifolium grown at different levels of Cu, Fe and Mn in a peat/sand substrate have been studied in relation to the micronutrient status of the plants and to the phenolase, peroxidase and IAA oxidase activities in the leaves. When iron levels were inadequate, bud formation was inhibited by the omission of copper but there was no effect of omitting copper when iron was fully adequate. Very few buds opened fully, however, wherever copper was omitted. The copper content of the plants grown without added copper (2.4 ppm) was well below the published ‘critical’ level and was unaffected by the other treatments, Phenolase activity in the leaves was markedly reduced by copper deficiency irrespective of the level of iron applied. When the level of copper was adequate, however, phenolase activity increased with the level of iron. Similar but less marked effects were observed with peroxidase and IAA oxidase. The significance of the results in relation to the mechanism of flowering in the chrysanthemum is discussed.