Cell-mediated immunity is important in host control of CMV infection. A chromium release microcytotoxicity assay was used to evaluate the role of cytotoxic T lymphocytes (CTL) in murine CMV infection. Within a few days after intranasal inoculation virus was detected in cultures of buffy-coat, spleens, anterior cervical lymph nodes and salivary glands. CTL were first detected on day 5 post-infection in spleen and peripheral blood, and on day 6 in anterior cervical nodes. The course of the CTL response approximated to that of virus titres during the acute phase of infection in the spleen and blood. The findings indicate that CTL are distributed to infected tissues and appear to be important during the acute, viraemic phase of infection.