We previously identified a translocation breakpoint in exon 8 of the erythropoietin receptor (EpoR) gene in TF-1 cells, a cell line derived from a human erythroleukemia. To investigate the potential pathogenetic significance of this abnormality, we more precisely mapped the breakpoint within exon 8 and studied the expression of the translocated gene by S1 nuclease mapping of EpoR transcripts and chemical crosslinking of labeled erythropoietin (Epo) to TF-1 cell surface receptors. Transcripts from the abnormal gene were found to be highly expressed in relation to normal EpoR transcripts in TF-1 cells. The breakpoint predicted by S1 mapping of abnormal EpoR transcripts agreed closely with that determined by Southern analysis. Chemical cross-linking of 125I-Epo to TF-1 cells showed an abnormal, low-molecular-weight cross-linked species directly recognized by anti-EpoR antibodies and present in considerable excess over the normal EpoR. Karyotype analysis showed that each of 10 TF-1 cell metaphases had, in addition to multiple other alterations, one chromosome 19 with additional chromosomal material translocated onto the short arm at 19p13.3, the location of the EpoR gene. We conclude that the structurally abnormal EpoR gene in TF-1 cells is highly expressed and produces an abnormal protein. We speculate that the chromosomal material brought into the EpoR locus by translocation is responsible for the high level of expression. We hypothesize that this translocation participated in the evolution of the erythroleukemia from which TF-1 cells were derived.