The cellular ets-1 gene homologous to the 5' region of the v-ets sequence of the E26 retrovirus codes for a 6.8-kb mRNA that is translated into a 51-kDa protein in human cells. A survey of mRNA from human tissues showed the thymus as the tissue with the highest level of ets-1 transcription, within other hematopoietic organs and tissues, including spleen, fetal liver, lymph nodes, bone marrow, and peripheral lymphocytes exhibiting low or undetectable levels of hybridization. A high level of ets-1 expression was found in murine thymocyte mRNA as well. Investigation of the ets-1 expression levels in human leukemic samples showed that primary malignant T cells (T-ALLs), corresponding to intrathymic stages of maturation, have a much higher level of ets-1 mRNA than malignant T lymphoid cells with a mature phenotype, such as adult T cell leukemias (ATLs). T-ALLs were also higher in ets-1 expression than the other lymphoid (pre-T-ALL, c-ALL, pre-B-ALL) malignant cells analyzed. Insignificant amounts of the specific ets-1 mRNA were detected in several acute myeloid leukemias representing various degrees of maturation. The elevated ets-1 mRNA in thymocytes suggests a biological role for the ets-1 product in these cells that could be explored to investigate ets-1 function. Finally, the exhibited expression of ets-1 in lymphoid cells and absence from malignant myeloid cells makes it a candidate marker for phenotyping human hematopoietic tumors.