Tumor-promoting phorbol esters affect a variety of cellular functions which may underlie tumor promotion. We isolated from human lymphocytes a cDNA clone whose gene is inducible by the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate as well as by the T-cell mitogen phytohemagglutinin. Nuclear transcription experiments suggested that this induction is primarily caused by the increased transcription of the gene. It is interesting that this gene is expressed constitutively in human T-cell leukemia virus-infected mature T cells. The results support the notion that 12-O-tetradecanoyl-phorbol-13-acetate can affect cellular functions by causing transcriptional activation of specific genes.