Tumors from female mice of 8 different strains were examined in thin section for B particles. These particles were found in all the tumors, although quantitative variations were noted among strains and from different levels within the tumors. There were characteristic localizations of virus particles in the interior and periphery of the tumors within individual mouse strains. The concentration of particles in tumors from nonlactating strain A mice was extremely low, but in tumors from lactating females of this strain the concentration was very high. Moreover, many particles found in tumors from strain A mice had a different internal structure from that of other strains. These variant particles were found also in hybrid back-crosses having strain A ancestors.