In previous studies, others and we have demonstrated by immunohistological methods in light microscopy that the prominent cell population infiltrating the psoriatic lesional dermis is represented by CD4-positive cells, thus suggesting an implication of T4 lymphocytes in the development of psoriatic lesion. However, recent findings have indicated that not only T4 lymphocytes, but also macrophage-like cells, are CD4-positive. In the present study, therefore, a highly specific and sensitive 'in situ' gold-immunoelectronmicroscopy procedure developed by us was performed to identify ultrastructurally with precision the nature of the CD4-positive cells infiltrating the dermis of the incipient psoriatic lesion. Although a consistent proportion (28%) of slightly CD4-positive macrophage-like cells was detected, the strongly CD4-positive lymphocytes were the prominent cell subset (51%). The important role presumably played by such CD4-positive lymphocytes in the pathogenesis of the psoriatic lesion is discussed. Our gold-immunoelectronmicroscopy technique improved the study of psoriatic cell subpopulations in situ, since (a) in comparison with immunocytochemistry in light microscopy, it allowed us not only to recognize the ultrastructure of the labelled cells, but also to detect even small amounts of antigen on the cell surface; and (b) in comparison with the commonly used peroxidase immunoelectronmicroscopy techniques, it allowed us to quantitatively assess the expression of antigens (e.g. CD4) on different cell subpopulations (e.g. the low CD4 expression by macrophage-like cells versus the high CD4 expression by T4 lymphocytes).