Investigations were performed in 6 cases of epidermodysplasia verruciformis and 2 healthy family members. Nonspecific cell-mediated immunity (CMI) was studied by measuring response to phytohemagglutinin (PHA) and concanavalin A (Con A), percentrages of E- and EAC-rosette-forming lymphocytes, bacterial skin tests, and allergic reactions to dinitrochloro-benzene (DNCB). Impairment of CMI was manifested by reduction in the percentage of E rosettes, and lowered response to PHA, and- to a lesser degree- to Con A. The immune response to DNCB sensitization was invariably negative. Impairment of CMI was greater in cases of long duration and with extensive lesions. The cases of similar duration and extent of lesions, which never showed tendency to tumor formation, were not different in CMI in comparison with cases with numerous tumors. Only in cases with very advanced tumors CMI was impaired parallel to the gravity of the patient's general condition.