The immunocompetent cells (B and T lymphocytes, mononuclear phagocytes, MPS cells) known to be responsible for the rejection of the HPV- (Human Papillomavirus) induced tumors in the skin (warts) were analyzed in the infiltrates of 63 oral squamous cell lesions of suspected HPV origin (Condyloma, CA; Focal Epithelial Hyperplasia, FEH; Squamous Cell Papilloma, SQP) as well as in tumors thought to be unrelated to HPV (Fibrous Hyperplasia, FH; Papillary Hyperplasia, PH; True Fibroma, TF; Keratoacanthoma, KA), using the ANAE (acid alpha-naphthyl acetate esterase) technique. All tumors were also stained by the indirect immunoperoxidase-PAP method for the demonstration of HPV antigens. Of the SQP's, 60% were HPV-positive, as were 80% of CA's and one third of FEH's. In addition, 1/18 FH's and 2/14 PH's stained HPV-positive. On electron microscopy, viral particles could be disclosed within the nuclei of HPV-lesions. The highest proportions of the T and MPS cells (the elements of cell-mediated immunity) were found in CA's and SQP's (15-17%), as compared with the values of less than 10% in all others. The infiltrates in FEH's also showed low values of MPS and T cells, interpreted to suggest an etiology of the HPV type (determining the type of immune reaction) different from that of CA and SQP. HPV-positivity in the lesions did not influence the distribution of the immunocompetent cells. The results indicate that immunological mechanisms are involved in HPV lesions of the oral cavity as they are at other sites. The role of HPV in the etiology of these (and possibly of the oral squamous cell carcinoma) was discussed. Further analyses of the lymphocyte subsets are needed to establish the role of host reactions in the development and eradication of HPV lesions in oral cavity.