An inhibitor of two ribopolymer-transcribing DNA polymerases can be isolated from MOPC-21 myeloma tumors, from the sera of mice bearing these tumors, or from MCDV-12 ascitic leukemia. The peritoneal fluid of mice bearing the latter neoplasm is also a source of the inhibitor. The inhibitor is a small DNA molecule which displays inhibitory activity in both double- and single-stranded form.
