Lymphocytes from seronegative and seropositive subjects were stimulated in vitro with herpes simplex, rubella, mumps and measles viral antigens. Viral antigens were beta-propiolactone inactivated crude material (containing cell-membrane fragments) grown in Vero cells and prepared identically. Lymphocytes from seropositive subjects responded specifically to herpes virus antigen, and most rubella and mumps seropositive subjects responded to the respective antigens. Measles antigen, however, did not stimulate lymphocytes from seropositive or seronegative subjects. The responses of three subjects studied repeatedly over a period of several weeks were reproducible. The results of the study support the usefulness of the blast transformation test as a measure of sensitization to viruses.