Persistent infection with lymphocytic choriomeningitis (LCM) virus activates a phenotypic expression of murine leukemia viruis-related antigen. NZB and (NZB x NZW)F(1) mice, which normally carry large amounts of Gross virus, and C57BL/6 and NZW mice, which normally carry little virus, were infected with LCM virus. All had Gross soluble antigen in their plasmas at 3 months of age, while noninfected matched controls of all strains did not. This effect was seen after infection with LCM virus that was tissue passed or plaque purified. Similarly, cultures of mouse-embryo fibroblasts produced Gross soluble antigen when infected with LCM virus, but noninfected cultures failed to do so.