Full thickness skin biopsies from four patients with borderline lepromatous leprosy (BL leprosy) have been examined. Immunohistological techniques have been employed to analyse the non-lymphoid mononuclear cells present in the dermal infiltrates associated with the BL lesions. This analysis was performed using three monoclonal antibodies, RFD2 (recognizing macrophages), RFD1 (recognizing interdigitating cells) and NA1/34 (recognizing Langerhans cells). It was found that the vast majority of non-lymphoid mononuclear cells in the lesions were RFD2+ macrophages. However, a significant number (15-30%) of macrophage like cells were RFD1+ interdigitating cells. A very small number of NA1/34+ Langerhans cells were also identified within the dermal infiltrates. Combination immunohistology and Ziehl Neelsen staining revealed that all these cell types could be found containing the Mycobacterium leprae organisms. The proportions of parasitized cells within each subpopulation was equivalent to the overall proportion of each cell type within the infiltrate. The significance of parasitism of cell types thought to be involved in antigen presentation and induction of immune responses is discussed.