Isocitrate lyase ( L s-isocitrate glyoxylate-lyase, EC 4.1.3.1) was present in the cotyledons of dry seeds of peanut and squash. Activity of the enzyme increased markedly in squash cotyledons during the first 3 days of germination, and over half the increase was under the control of axial tissue. The presence of puromycin in the culture solution severely inhibited formation of isocitrate lyase, indicating partial de novo synthesis of the latter. The presence of benzyladenine, a cytokinin, in the culture solution could replace, in part, the hormonal stimulus produced by the embryonic axis and which normally is required for maximum development of isocitrate lyase activity in the cotyledons. Gibberellin at 10 μM concentration was incapable of replacing this stimulus. Endothall, 100 μM, inhibited isocitrate lyase development stimulated by the presence of the embryonic axis.