SUMMARYT and B lymphocyte populations were evaluated in forty‐one patients with multiple recurring warts. Active rosettes (T‐Ea) and total T lymphocytes (T‐Et) were significantly decreased.Thein vitroeffect of a calf thymus extract (TP‐1) on peripheral blood lymphocytes was evaluated in thirty‐three patients. A statistically significant increase in E‐rosette forming cells was observed in twenty‐three patients (69%). Serum blocking factors were not found in the patients with a positive response to TP‐1, which suggests the possibility of a thymic hormone deficiency. In the case of non‐responders to TP‐1, on the other hand, an anti‐lymphocyte antibody capable of directly blocking the hormone receptors on the undifferentiated lymphocy