The hemoglobin beta-1 gene of the rabbit was linked to a 244 bp DNA fragment from the beginning of the polyoma virus late region, not including the viral origin of replication. After transfection of such recombinant DNAs into mouse 3T6 and human HeLa cells, the polyoma sequences were found to strongly enhance the level of correct beta-globin gene transcripts over a distance of at least 1400 bp. These findings are similar to those obtained with a segment of DNA from the corresponding region of the SV40 genome (J. Banerji, S. Rusconi and W. Schaffner, 1981, Cell, in press) which, however, shows very limited sequence homology to the polyoma 244 bp segment. Using the same assay, a complete copy of polyoma virus DNA was found to interfere with the enhancement of globin gene expression in a cell type-specific manner which may be due to incorrect transcription. In contrast to the complete polyoma virus genome, the 244 bp DNA fragment will be particularly useful as a component of mammalian expression vectors since it almost exclusively yielded high levels of correct beta-globin gene transcripts.