Respiratory challenge of C57BL/6 mice with murine gammaherpesvirus 68 induces proliferation of T lymphocytes early after infection, as evidenced by incorporation of the DNA precursor bromodeoxyuridine. Using pulse-chase analysis, splenic and peripheral blood activated T lymphocytes were found to continue dividing for at least a month after the initial virus challenge. The results are in accord with the idea that T cells are stimulated for a substantial time after the acute, lytic phase of virus infection is resolved.