The therapy with thymus extracts, oral and parenteral, reduced the frequency and the intensity of recurrences of herpes labialis infections. The duration of the effect was at least 6 month after interruption of the therapy. Indomethacin was effective and developed and intensive effect only during the therapy. The skin test with recall-antigens and the neopterin-elimination were altered at the first day of the menstruation during the recurrence. The normalisation succeeded during therapy. In patients with recurrences in the perimenstrual time we observed a reduced T-helper/T-suppressor index during the first day of menstruation. Normal data were registered out of the recurrence time and/or under therapy. Inhibitors of the lymphokine: leucocyte/migration inhibitory factor (LIF) with a molecular weight of 6-12 KD were obtained with the specific stimulation of mononuclear cells of patients with recurrent infections with herpes labialis using the herpes-virus-1 antigen. The inhibition of fibrinolysis/proteolysis with aprotinin, tranexamic acid, phenyl-methyl-sulphonyl-fluoride and di-isopropyl-fluorophosphate could prevent the appearance of inhibitors. Inhibitors could be produced by splitting the LIF-molecule with urokinase and plasminogen but not with trypsin. The production, but not the activity, of present LIF-inhibitors are blocked in vivo and in vitro by indomethacin and thymus peptides.