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Three laboratories previously have obtained from Zea mays kernels (milk stage) one or more 6-(substituted)aminopurines capable of greatly promoting cell division in plant tissue cultures.1-3 At least two of the laboratories have worked on a single compound4 which has been identified as 6-(4-hydroxy-3-methylbut-2-enyl)aminopurine.5 The name zeatin has been applied to this particular compound.6 Only a small portion of the total cell-division activity in crude extracts from kernels seems to be due to zeatin'-3 and its actual existence in the kernel has been questioned.7 We now present evidence strongly indicating that zeatin does occur naturally in its unsubstituted form and also in nucleoside and nucleotide forms. Assay.-The active materials in the maize extracts were detected by using the soybean (Glycine max, var. Acme) tissue test.3' 8 This tissue requires a substance like zeatin for continued cell division and has proved to be very sensitive to zeatin and similar compounds. For example, in recent tests we have obtained measurable growth responses to as little as 5 X 10-11 M zeatin. Most assays were performed with small quantities of extracts which contained rather low concentrations of active materials. With such low concentrations, the soybean test shows considerable variation of fresh weights for individual pieces in a particular assay but still a high degree of reproducibility from test to test. Repeats of all experiments reported here have given consistent results. Stock cultures of the soybean tissue have been maintained on a medium containing (mg/liter

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