The RET proto-oncogene product is a receptor tyrosine kinase representing the signal-transducing molecule of a multi-subunit membrane receptor complex for at least two different types of transforming growth factor (TGF)-beta-related neurotrophic factors. We have previously shown that RET gene expression in acute myeloid leukemia (AML) occurs more frequently in AMLs displaying either a monocytic (FAB M4/M5) or intermediate-mature myeloid phenotype (FAB M2/M3) than in leukemias reflecting an earlier stage of myeloid differentiation (FAB M0/M1). To further verify the association between RET expression and the relative maturation stage of AML cells, we have performed a quantitative estimation of relative abundances of RET transcripts among various FAB subtypes of AMLs. By analyzing 13 AML samples and normal hematopoietic cells through a competitive-quantitative RT-PCR approach, we were able to show that the relative levels of RET-specific mRNAs continuously increase with blast cell maturation in human AML, i.e., the amounts of RET gene-specific transcripts differ among RET-expressing AMLs, being higher in the more differentiated FAB phenotypes. In addition, we provide evidence that the relative amounts of RET transcripts increase upon in vitro and in vivo differentiation of leukemic promyelocytes from FAB M3 AML patients, becoming overall comparable to those found in normal granulocytes. These results indicate that RET expression in human AMLs is maturation-associated, probably mirroring the developmental regulation of this gene during differentiation of normal hematopoietic cells.