The present paper describes experiments conducted over the last several years which have focused primarily on the development of mammary tumors in BALB/c mice after neutron and gamma irradiation. Time-dose relationships for induction of mammary and lung tumors following irradiation with fission-spectrum neutrons and 137Cs gamma rays are described. Subsequent studies have used in vivo/in vitro approaches to characterize cellular and molecular changes. These studies suggest quantitative and qualitative differences in the effects of neutrons and gamma rays.