Cytokinins are important purine derivatives that act as redifferentiation-inducing hormones to control many processes in plants. Cytokinins such as isopentenyladenine (IPA) and kinetin are very effective at inducing the granulocytic differentiation of human myeloid leukemia HL-60 cells. We examined the gene expression profiles associated with exposure to IPA using cDNA microarrays and compared the results with those obtained with other inducers of differentiation, such as all-trans retinoic acid (ATRA), 1 alpha,25-dihydroxyvitamin D3 (VD3) and cotylenin A (CN-A). Many genes were up-regulated, and only a small fraction were down-regulated, upon exposure to the inducers. IPA and CN-A, but not ATRA or VD3, immediately induced the expression of mRNA for the calcium-binding protein S100P. The up-regulation of S100P was confirmed at the protein expression level. We also examined the expression of other S100 proteins, including S100A8, S100A9 and S100A12, and found that IPA preferentially up-regulated S100P at the early stages of differentiation. IPA-induced differentiation of HL-60 cells was suppressed by treatment with antisense oligonucleotides against S100P, suggesting that S100P plays an important role in cell differentiation.