DNA unprotected by protamine immediately degraded to fragments of a 200 bp size in the blood flow of experimental animals to be delivered to the innermost compartments of tumor cells. Once absorbed by protamine, DNA fragment remained unchanged in size. For intravenous injection of 1-2 microg, the amount delivered to the cells of intramuscularly grafted tumor RLS was well below several copies on a basis of 1,000 bp. There was no correlation between the amount delivered and DNA-protamine linkage. Protamine protection did not affect DNA's ability to inhibit experimental tumors.