In vitro lymphocyte reactivity (LR) to cytomegalovirus (CMV)-infected human fetal fibroblasts (CMVFF) and cell-free CMV were measured by using lymphocytes from healthy donors. Lymphocytes from all seropositive donors were stimulated by CMVFF, whereas lymphocytes from negative donors were not. The optimal stimulator cell-to-lymphocyte ratio was in the range of 1:5 to 1:50, dependent on the virus dose used. LR to cell-free CMV was positive for 15 out of 18 seropositive donors and negative for 14 out of 16 seronegative donors. In most cases LR to CMVFF was considerably higher than LR to cell-free CMV. Within the CMV seropositive group there was no significant correlation between the LR to either CMVFF or cell-free CMV and the levels of antibodies to CMV early antigens or CMV late antigens. There was no strict correlation between LR to CMVFF and to cell-free CMV, especially not in tests with lymphocytes from two patients with CMV mononucleosis. Our data suggest that CMVFF and cell-free CMV are recognized (partly) by different subpopulations of CMV-specific memory lymphocytes. We conclude that the use of CMV-infected cells, in addition to cell-free CMV, in LR tests gives more reproducible and possibly also additional information about CMV-specific cellular immunity.