Lymphocytes from 19 healthy donors were tested against purified cytomegalovirus (CMV) strain Ad 169, in a lymphocyte-transformation test. The test was performed in microcultures using various preparations and concentrations of antigen. These included, besides purified CMV, CMV-infected cells, herpes simplex virus type 1 antigen, and control antigens. Lymphocytes from CMV-seropositive donors were stimulated by purified virus and infected cells, whereas lymphocytes from seronegative donors did not respond. Similarly, only lymphocytes from herpes-seropositive donors did respond to herpes antigen. With these preparations of antigen the test was found to be sensitive and specific.