We have investigated stages of thymic lymphoma development in radiation and N-methylnitrosourea (NMU)-treated C57BL/6J mice. The lymphoma cell was identified serologically as a cortical population bearing MEL-14hi, H-2Khi, and IL-2R+ surface markers. According to these parameters in C57BL/6J mice the lymphoma cell was the same regardless of inducing agent or activated oncogene (ras or non-ras). Transforming activity in the radiation and NMU-induced tumors was analyzed using both the nude mouse tumorigenicity assay and the focus-forming assay. 8/10 NMU-induced tumors and 12/15 radiation-induced tumors showed transforming activity in the tumorigenicity assay. Southern blot analysis of the nude mouse transformants demonstrated K-ras transforming sequences in eight of eight NMU-induced lymphoma DNAs, two of 12 radiation-induced lymphoma DNAs and N-ras transforming sequences in five of 12 radiation-induced lymphoma DNAs. The non-ras transforming activity in five DNAs from radiation-induced thymic lymphomas indicates the presence of an unidentified oncogene(s) in these tumors. Staging of thymic lymphoma development in this animal model system will allow the study of oncogene activation early in the course of carcinogen-induced disease. These results also emphasize the high sensitivity of the nude mouse assay to score for activated oncogenes and might also indicate a high frequency of K-ras activation in NMU-induced lymphomas in C57BL/6J mice.