The prevalence and distribution of lymphocyte subpopulations in normal and acanthotic ovine skin were investigated using monoclonal antibody immunocytochemistry. CD8 + cells were predominant in the epidermis of both normal and acanthotic skin, but were CD8 + cells, CD4 + cells and T19 + cells infrequent in normal epidermis. Within the dermis of normal skin, there were significantly greater numbers of CD4 + and T19 + cells situated around the superficial dermal vessels than in any other region examined. The majority of the CD8 + cells adjoined vessels, but the proportion that did not was greater for CD8 + than for CD4 + or T19 + cells. The CD4 + and CD8 + subsets were represented equally in adnexa. T cells were of memory phenotype. B cells and naive T cells, both of which express the CD45RA antigen, were rarely seen and tended to be associated with vessels in both normal and acanthotic skin. None of the T19 + cells (which are γδ + ) resembled the dendritic γδ cells seen in murine epidermis. Acanthotic skin was strikingly different to normal skin. There was a greater abundance of T cells, particularly CD4 + cells, in acanthotic epidermis and the numbers of CD8 + and T19 + cells, and to a greater extent CD4 + cells, were greater at the dermal-epidermal junction. There were more CD4 + and CD8 + cells in the superficial dermal stroma of acanthotic skin. Within the dermis of acanthotic skin, T cells were concentrated near vessels but the apportioning of T cells between stromal/adnexal and vessel-associated sites differed from normal. Such observations suggest that migration away from perivascular sites and into the stroma may be controlled separately for subregions of skin and for each T cell subset. The role of this altered non-random migration of T cells in skin chronically exposed to ultra violet radiation is uncertain.