The incorporation of mouse S-EAC DNA into homologous normal cells (mouse embryo secondary cultures), and into heterologous cancer cells (TC-SV40 line), with both systems having their native DNA blocked by BrUdR incorporation, was studied. 3H-TdR-DNA was inoculated with DEAE-D to protect it and to potentiate its incorporation, the process being autoradiograohically controlled. The amount of incorporated DNA was radioisotopically determined, and the incorporation process was studied by analysing the fractions obtained after density gradient centrifugation separation of the inoculated cells DNA. Receptivity was greater in those cells inoculated with DEAE-D-protected DNA. The incorporation was slightly greater for cells whose DNA had been blocked by BrUdR incorporation, and for homologous with respect to heterologous cells. In those cells inoculated while the DNA blockade was incomplete, part of the inoculated DNA became incorporated into the cell genome (L-H chains). However, in the completely blocked cells it could not be determined if the incorporation occurred in a lysogenic-like or in an episomic-like form.