Several lines of evidence from in vivo animal experiments and human studies suggest that leptin, a peptide secreted from adipose tissue, plays a role in regulating food intake and energy expenditure. However, the signal transduction mechanism of leptin in its target cells remains unknown thus far since leptin-responsive cell lines have not been available yet. We found that leptin caused the tyrosine phosphorylation of several proteins in human renal cell carcinoma cells, ACHN cells, in which STAT-1, but neither STAT-3 nor STAT-5, was involved. An ACHN cell line would serve as a useful tool for analyzing the signal transduction mechanism of leptin.