Summary: Tumors on both animals and plants arise as a result of loss of growth control, and in animal systems have been a source of many insights into regulatory mechanisms. We have attempted to develop a similar paradigm in plants by using ionizing radiation to induce tumors inArabidopsis thaliana. From the primary tumors we have established a number of hormone autonomous tissue lines in culture which present a variety of phenotypes: leafy, rooty, and undifferentiated. These lines also differ from one another in their hormone content and in their response to exogenous hormones. Some tumor tissue lines have higher levels of free or conjugated auxin or cytokinin, whereas others do not. Some lines show the same growth response as normal callus when hormones are supplied in the growth medium, whereas others have increased or decreased sensitivity to hormones. Using subtraction hybridization, we have identified a number of cDNAs whose expression is enhanced in the tumors compared with hormone-dependent callus. Several of these cDNAs have been identified by homology with sequences in Genbank, and include a membrane channel protein, a lipid transfer protein and hydroxyproline-and glycine-rich proteins. The expression of these relatively abundant proteins may well be a consequence rather than a cause of tumorigenicity. Some of the less abundant transcripts remain unidentified. Two of the tumors described have shown secondary changes in culture, leading to faster-growing cell lines. In one case this was associated with an increase in free indole-3-acetic acid (IAA), a reduction in IAA conjugates and an increased sensitivity to naphthylphthalamic acid. In a second instance, the primary tumor had increased levels of cytokinin but was cytokinin insensitive while the fast-growing variants retained high cytokinin levels, had become cytokinin sensitive and acquired a shooty phenotype. Ways in which this approach to understanding plant hormones and growth control may be developed are discussed.