The expression patterns of a cDNA clone, p48h-10, of an auxin-induced gene were examined in isolated mesophyll cells of Zinnia and in the organs of Zinnia plants. In the isolated mesophyll cells, the mRNA accumulates in 48 hr of culture with 1-naphthaleneacetic acid alone. Because the first cell division occurs before 36 hr of culture, the gene probably is not involved in cell division. Benzyladenine does not induce expression of this gene, but the combination of 1-naphthaleneacetic acid and benzyladenine induces the mRNA accumulation about 24 hr earlier than does 1-naphthaleneacetic acid alone. Tissue print hybridization shows that the mRNA is present predominantly in the cambial region in stems, leaves, and roots and in the vascular bundles in flower buds but does not occur in the apical regions of shoot or root. The characteristics of the gene expression, including auxin- and cytokinin-regulated induction and cambial region localization, encourage us to suggest that the gene is involved in the early process of vascular differentiation.