The transcription factor GATA-1 is required for the normal development of erythroid cells. GATA-1 is also expressed in other hemopoietic cells, suggesting that it might be initially activated in a multipotent progenitor. To immortalize GATA-1-expressing progenitors, we generated mice transgenic for a thermosensitive SV40 T gene, driven by the GATA-1 promoter-enhancer. Immortalized marrow cells grow in culture at 32 degrees C but not at 38 degrees C, and are dependent on erythropoietin (Epo) or interleukin 3 (IL-3). Epo dependent cells express hemoglobin, high levels of GATA-1, GATA-2 and NF-E2 p45 mRNAs, and are positive for stem cell antigen 2 (Sca-2) and the early myeloid marker ER-MP12. IL-3 dependent cells can be derived from Epo dependent lines, and are hemoglobin-, Sca-2- and ER-MP12-negative, have low GATA-1 and NF-E2 p45 mRNA levels, and express myeloid markers Mac-1, F4/80 and Gr-1. Brief treatment of Epo dependent cells with myeloid growth factors (plus Epo) leads to the induction of Mac-1, F4/80 and Gr-1, concomitant with the disappearance from most cells of Sca-2, ER-MP12 and GATA-1 driven T antigen nuclear expression. Thus, the immortalized Epo dependent cells have the property of a progenitor capable of differentiation towards either the erythroid or myeloid lineages. These cells initiate transcription of a proportion of GATA-1 RNA molecules at an upstream promoter, previously known to be expressed only in testis cells.