Seaweed extract, prepared by alkaline extraction of Ascophyllum nodosum (L.) Le Jol., applied either to the soil or to the foliage of tomato plants, produced leaves with higher chlorophyll levels than those of control plants. The effects on leaf chlorophyll content were investigated using a cucumber bioassay procedure devised for cytokinins. The seaweed extract was shown to increase the chlorophyll levels of the cucumber cotyledons, but 'peaks' of activity were obtained when widely different concentrations were used. The possibility that these effects were the result of betaines present in the extract was considered. Glycinebetaine, y-aminobutyric acid betaine and 6-aminovaleric acid betaine all produced significantly enhanced chlorophyll concentrations in the cotyledons. 'Peaks' of activity were observed for each betaine: for glycinebetaine at 10-6 and between 10-4 and 10' mg 1-', for y-aminobutyric acid betaine at 10-6, between 10-4 and 10 -1, and 10' mg -, and for 6-aininovaleric acid betaine between 10-5 and 10 mg 1-1. It was concluded that the effects of enhancing chlorophyll levels produced by the seaweed extract were due, at least in part, to betaines.