Recently, we have described a monoclonal antibody, named MS-1, which identifies a novel high-molecular-weight protein expressed by noncontinuous, sinusoidal endothelia and by interstitial dendritic cells in certain normal human organs (S Goerdt, LJ Walsh, GF Murphy, JS Pober, J Cell Biol 1991, 113:1425-1437; and LJ Walsh, S Goerdt, JS Pober, H Sueki, GF Murphy, Lab Invest 1991, 65: 732-741). In this report, we demonstrate in studying a variety of skin lesions that MS-1 antigen can also be expressed by endothelia of continuous origin under certain pathological conditions. Among the skin lesions tested, MS-1 antigen expression by endothelial cells of continuous origin is frequently observed in wound healing tissue, in cutaneous T-cell lymphoma, in psoriasis, and in melanoma metastasis, ie, in 100%, 80%, 71%, and 71% of cases, respectively. In contrast, endothelial MS-1 antigen expression rarely occurred in other skin lesions, including vascular tumors, six of which were Kaposi's sarcomas (13% and 0% of cases with vascular MS-1 expression, respectively). The percentage of cases with MS-1+ vessels is only marginally different in malignant versus benign lesions (55% versus 31%); when melanocytic nevi, primary melanomas, and melanoma metastases are compared, however, an increase in the percentage of cases with MS-1+ vessels is seen (31%, 50%, and 71%, respectively). Apart from wound healing, the relative number of MS-1+ vessels in a given lesion amounts to less than 5% compared with the number of continuous type vessels stained by monoclonal antibody 1F10 (S Goerdt, F Steckel, K Schulze-Osthoff, H-H Hagemeier, E Macher, C Sorg, Exp Cell Biol 1989, 57: 185-192). In addition, the occurrence of MS-1+ vessels is not related to the overall vascularity of a given lesion. Thus, the conditions for MS-1 antigen expression by endothelia of continuous origin cannot as yet be exactly defined. Furthermore, we have noticed that the number of MS-1+ dendritic cells varies considerably in skin lesions; in the early patch lesions of Kaposi's sarcoma and in juvenile xanthogranuloma MS-1+ cells even constitute the major cell type. This prompted us to investigate MS-1 antigen expression and its regulation in cultured human monocytes/macrophages. Expression of MS-1 antigen by these cells regularly starts at day 3 of culture and reaches its maximal value at day 9, after which it declines.(ABSTRACT TRUNCATED AT 400 WORDS)