Each of the two human genes encoding the α and β subunits of a heterodimeric transcription factor, PEBP2, has been found at the breakpoints of two characteristic chromosome translocations associated with acute myeloid leukemia, suggesting that they are candidate proto-oncogenes. Polyclonal antibodies against the α and β subunits of PEBP2 were raised in rabbits and hamsters. Immunofluorescence labeling of NIH 3T3 cells transfected with PEBP2α and-β cDNAs revealed that the full-size α A1 and α B1 proteins, the products of two related but distinct genes, are located in the nucleus, while the β subunit is localized to the cytoplasm. Deletion analysis demonstrated that there are two regions in α A1 responsible for nuclear accumulation of the protein: one mapped in the region between amino acids 221 and 513, and the other mapped in the Runt domain (amino acids 94 to 221) harboring the DNA-binding and the heterodimerizing activities. When the full-size α A1 and β proteins are coexpressed in a single cell, the former is present in the nucleus and the latter still remains in the cytoplasm. However, the N-or C-terminally truncated α A1 proteins devoid of the region upstream or downstream of the Runt domain colocalized with the β protein in the nucleus. In these cases, the β protein appeared to be translocated into the nucleus passively by binding to α A1. The chimeric protein containing the β protein at the N-terminal region generated as a result of the inversion of chromosome 16 colocalized with α A1 to the nucleus more readily than the normal β protein. The implications of these results in relation to leukemogenesis are discussed.