Although previous studies have suggested that human peripheral blood mononuclear cells (PBMCs) may express pro-opiomelanocortin (POMC) mRNA and synthesize its related peptides, the patho-physiological role of POMC expressed in peripheral cells is not known. In this study, we investigated the POMC gene expression in various types of human leukemia cell lines by Northern blot analysis and the reverse transcribed-polymerase chain reaction (RT-PCR) method. The POMC mRNA was not detected by Northern blot analysis in all cell lines tested except the Jurkat cell line which is derived from T-lymphoblastic leukemia. The POMC mRNA expressed in the Jurkat cells was smaller than that in the human anterior pituitary gland. The RT-PCR method revealed that a truncated-POMC transcript could be detected not only in lymphoblastic leukemia cells but also in erythroid and myeloid cells. Interestingly, two cell lines of monocytic leukemia, J-111 and U937, did not express the truncated-POMC mRNA. Treatment with concanavalin-A stimulated truncated POMC mRNA expression and ACTH-like immunoreactivity in lymphoblastic leukemia cells with T-(Jurkat) and B-(BALL-1) lymphocyte phenotypes. These results confirm that human leukemia cells except for monocytic cells express a truncated-POMC mRNA as well as in the human normal PBMC.