We have activated a thymus-derived fibroblast cell line from 129/J mice for production of noninfectious type C viral particles. Retroviral particles were produced continuously after cells were treated with ethylnitrosourea, a powerful mutagen and carcinogen. Antigenic analysis of the major structural protein of these particles showed a close relationship with an infectious virus from Mus caroli, a potential progenitor of the horizontally transmitted primate type C viruses. This finding demonstrates the potential of cells from inbred strains of Mus musculus to express upon mutagenesis antigens of the Mus caroli class of endogenous MuLV.