The dynamics of intraepidermal penetration of the scabies mite and the host's responses to the invaders have been studied in hyperkeratotic scabies. Contrary to the generally accepted view, the mites progressed not only through the stratum corneum but also into the underlying epidermis and even occasionally to the dermo-epidermal interface. Morphological evidence of reactive processes included parakeratosis surrounding the burrows, psoriasiform hyperplasia with increased basal cell labelling indices and exocytosis of mononuclear cells. 3H-Thymidine labelling indices of immunocompetent cells of the dermal infiltrate reached levels found in allergic patch test reactions and were in both cases higher than in non-immune-specific inflammatory processes. The labelling index of peripheral blood lymphocytes showed an increased ratio of B cells to T cells as compared to controls or patients with allergic contact dermatitis. Serum CIq fixation was higher than in untreated patients with common scabies. These results support our previous findings of immunological activation in host infested by the scabies mite.