Infection of adult BALB/c mice with murine leukemia virus (MuLV) induces thymic lymphomas histologically indistinguishable from those caused by neonatal infection. X-irradiation permitted early and high levels of viral expression when given before or after MuLV administration and hastened the development of lymphomas. Expression of virus was assayed by using a radioimmune assay for murine p30, a virion core protein. Seventeen to 21 days after injection of MuLV into adult mice, there was 0.3 mug p30 per ml serum, approximately 5 times normal. Seventeen to 21 days after injection of MuLV into X-irradiated (600R) adult mice, there were 2.7 mug p30 per ml serum. The virus produced by infected adult mice was infectious and oncogenic when given to newborn mice.