As shown below, isolated fenugreek (Trigonella foenum graecum L.) cotyledons respond readily to cytokinins with increased expansion. Similar responses have been described for isolated cotyledons of spinach (4) and radish (5) and for sections of Xanthium cotyledons (3). A comparison shows that the fenugreek cotyledons exhibit a higher degree of specificity as they do not respond to GA. The findings may lead to an alternative bioassay of cytokinins. The use of cotyledons has a number of advantages from the point of simplicity in laboratory practice. Also cotyledons exhibiting cytokinin specific responses may be used to investigate the biochemical mechanism of cytokinin action as we report elsewhere (8). Here we describe the expansion of isolated cotyledons as affected by some growth substances, light, nitrogenous compounds, sucrose, and dormancy status. As in other Trifolieae, the attached cotyledons do not merely function as a reserve store, but expand and become green in light. The cotyledons can be separated from the embryonic axis by a cut through the dry seed as the space between these parts is indicated by a groove in the seed exterior. Further manipulationssterilization and washing of cut seeds, their transfer onto wet filter paper in Petri dishes, removal of seed coats 20 hr later, weighing and layering of cotyledons-were done in green safe light. Usually five cotyledons were arranged in a 7-cm Petri dish containing one filter paper disc to which was added 3 ml of test solution containing 1 mg each of mycostatin (nystatin), penicillin, and streptomycin per 50 ml while four to six replicates constituted one treatment. For light treatments the dishes were exposed to continuous incandescent light (30 ft-c near dishes) at 25 C. The dark treatments were enclosed in light tight boxes at the same temperature. CO2 treatment was given by incubating an uncovered Petri dish containing the cut seeds during the initial 20-hr soaking period for 18 hr in a desiccator in which CO2 was released from sodium bicarbonate by hydrochloric acid. To minimize variability due to differences in initial size of cotyledons, results have been expressed as relative increases (%) in fresh weight. Thus, within one experiment equal kinetininduced expansion was observed for samples of seed differing close to 100% in average air dry weight. Figure 1 shows the effect of exposing cotyledons in light to 5 X 10-5 M kinetin for increasing periods of time. In another experiment incubation under identical conditions but in the presence of 20 mM KNO3 resulted in a statistically significant growth stimulation by kinetin after 12 hr. Table I gives the effects of some cytokinins at different concentrations in darkness and in light after 48 hr. Light approximately doubled the growth responses. The lowest concentration at which kinetin gave in light a statistically significant response was 5 X 10-7 M and for 6-benzylade-