The skin equivalent (SE) has been validated as a model for studies on proliferation of keratinocytes. SEs were prepared from normal skin by implanting punch biopsies on dermal equivalents consisting of fibroblasts in a collagen matrix. The outgrowths were measured by planimetry. An immunohistochemical investigation with antibodies against markers associated with proliferation was performed on frozen sections from SEs during outgrowth at 3-6 days (SE6) as well as after completion of outgrowth at 21 days (SE21). Biopsies from normal controls and from uninvolved and involved skin in psoriatic patients were also studied. The antibodies used were Ki-67, cytokeratin 8.12, and antibodies against the receptors for epidermal growth factor (EGFr) and transferrin (TFr). The increase in area was linear during the first 7 days of culture and usually reached the edges of the dermal equivalent at this time. In SE6 TFr was expressed in the basal part of the outgrowth while the other markers were not observed. In SE21 and in psoriasis there was abundant epidermal staining of Ki-67-positive nuclei and cytokeratin 8.12 was detected in the suprabasal part of the epidermis. EGFr and TFr were seen in the basal layer in SE21. In the psoriatic lesions these receptors were found both in the basal and suprabasal layers. The lack of proliferation markers in SE6 indicates that the initial increase in the area of keratinocytes is due to migration from the punch biopsies. Increased cell proliferation is present in SE21, a finding in common with psoriasis and wound healing. The skin equivalents should therefore be an appropriate model for studies on these phenomena.