The distribution of Langerhans' cells and T and B lymphocytes in cervical epithelium affected by wart virus infection (WVI) and intraepithelial neoplasia (CIN) has been studied by immunohistological techniques with a panel of monoclonal antibodies. In comparison with normal tissue epithelium infected by wart virus consistently showed a partial depletion or absence of both Langerhans' cells and T lymphocytes. The few Langerhans' cells which remained showed loss of their normal dendritic processes and were confined to the vicinity of the basement membrane. In contrast, epithelium from patients with CIN showed an increase in spindle-shaped Langerhans' cells associated with increased numbers of stromal and intraepithelial lymphoid cells. These findings suggest that there is a marked difference in host immune responses in these two conditions and may explain why WVI tends to persist in the epithelium while CIN is often associated with a lymphoid reaction.