The maintenance of telomere length is crucial for the survival of cells. Recently, genes for proteins that consist of human telomerase have been cloned and the results have indicated a close relationship between telomerase activity and its gene expression. We studied the mRNA expression of the telomerase-associated genes, hTERT and TEP1, in hematopoietic cells in order to clarify the relation between them and telomerase activity using semiquantitative RT-PCR. In polymorphonuclear cells and monocytes isolated from peripheral blood, which had no detectable telomerase activity, no hTERT mRNA expression was seen. On the other hand, lymphocytes and CD34-positive cells both demonstrated hTERT mRNA expression. TEP1 mRNA was detected in all samples, showing no differential expression. We then assessed hTERT and TEP1 mRNA expression in CD34-positive cells cultured in vitro with growth factors. After 4 weeks of culture, all the cells showed myeloid differentiation and the telomerase activity was downregulated. hTERT mRNA was expressed in CD34-positive cells, but was downregulated in 4-week-cultured cells. TEP1 showed no apparent differential expression. We conclude that hTERT mRNA expression is downregulated in accordance with telomerase downregulation during the course of myeloid differentiation, which suggests that it plays a crucial role in the expression of enzyme activity, while TEP1 has a much smaller role to play, if any.