The distribution of peripheral blood T and B lymphocytes, the in vitro lymphocyte response to PHA, and in vivo experimental DNCB sensitization were studied in patients with different clinical forms of warts (common, 84; flat, 88; plantar, 22; genital, 14) and in 15 cases of epidermodysplasia verruciformis (EV). The percentage of T lymphocytes forming E rosettes was significantly decreased in patients with common (54.8%), flat (47.5%) and plantar (58.3%) warts, and those with EV (47.6%) in comparison with normal controls (68.4%). The DNCB sensitivity developed less frequently and it was less intensive in patients with common and flat warts than in the normal population. 60% of EV cases were anergic to challenging doses of DNCB. The lymphocyte response to PHA was reduced in all groups of patients studied as compared to normals. T cell function was found to be most defective in patients with EV and those with flat warts. Only a slight but statistically significant defect was demonstrated in the common wart group. CMI in patients with both plantar and genital warts was shown to be almost normal; except minor alterations of PHA-induced lymphocyte transformation and E rosetting T lymphocyte counts. These data have shown the divergency of CMI defect in the patients with different clinical forms of warts caused by various HPV types. This could indicate that distinct HPV types varied in their infectiveness and host cell-mediated resistance is a fundamental factor preventing viral infection.