Spontaneous regressions of papillomavirus lesions frequently occur in both human and animal infections. The mechanism by which this occurs is currently unknown. Mononuclear infiltrates are found in regressing human and rabbit papillomas. To assess the potential functional role of these infiltrates in regression, we have characterized and quantitated the cell types present in regressing rabbit lesions. Forty New Zealand white rabbits were inoculated with cottontail rabbit papillomavirus (CRPV) at 2 sites on the dorsal skin. All tumors on 6 rabbits markedly decreased in volume within 6 to 8 weeks of inoculation. Tumors on 4 of these 6 regressor rabbits were studied by immunohistochemistry. Regressor papillomas had conspicuous leucocytic infiltrates, most concentrated at the epithelial basement membrane, and often obliterating the basal cells of the germinal layer. Infiltrating leucocytes were also concentrated in the subjacent dermis immediately beneath the basement membrane. The infiltrates gradually lessened at increased depths in the dermis. In contrast, progressor papillomas contained fewer leucocytes, which were randomly distributed in the dermis. The phenotype of the infiltrating leucocytes was examined in 4 regressing and 12 progressing papillomas. In regressing papillomas, infiltrating leucocytes were predominantly T cells (68.0%), with relatively few B cells (7.4%). Progressing papilloma dermis contained fewer T cells and B cells than regressing papillomas. Most of the infiltrating T cells in regressing papillomas were labelled with a rabbit MHC-class-II-specific monoclonal antibody (MAb) (2C4), in contrast to only a small number in progressing papillomas. In addition to the leucocytic infiltrates, keratinocytes in regressing, but not in progressing, papillomas, frequently exhibited strong 2C4 staining. These results demonstrate that infiltration with T cells expressing rabbit class II is characteristic of regressing Shope papillomas and strengthens the assertion that cell-mediated immunity is the mechanism of Shope papilloma regression.