Thirty patients with recurrent herpes simplex and 6 controls were included in our study. Before initiation of treatment, during therapy with Levamisole, BCG, poison ivy and herpes antigen vaccine and, thereafter, MIF-determinations were performed. The latter test proved to be a useful parameter in evaluating the sensitivity against herpes infection, (and subsequent recurrent manifestations), and effect of therapy. Treatment with Levamisole, BCG and herpes antigen vaccine was successful. MIF-inhibition values paralleled the therapeutic effect in the Levamisole- and herpes antigen vaccine treated group, not however, in the BCG-treated patients