Immunologic cytotoxicity is an important endpoint of the immune response to tumors, viral infected cells, grafted tissues, and exogenous microorganisms, and is also an important mechanism of disease, especially in autoimmunity. There are multiple mechanisms of immunologic cytotoxicity, but each has three major stages: leukocyte/target attachment, specific recognition, and target lysis following effector activation. Adhesion molecules present on leukocytes and potential targets appear to be involved in all three stages of cytotoxicity. A major factor in all types of cellular cytotoxicity is the interaction of LFA-1 on leukocytes and CAM-1 on targets. Modulation of ICAM-1 levels on target by the cytokines TFN-g, IL-1, and TNF-a is a major point of control of the susceptibility of targets to cytotoxicity by many different cytotoxic mechanisms. It also appears that modulation of the avidity of LFA/ICAM-1 binding is another important control point in modulating immunologic cytotoxicity. Cytokines also have important effects on immunologic cytotoxicity in ways other than adhesion molecule induction: effector priming to better respond to specific recognition signals, effector mobilization into tissue, and expansion of cytotoxic effector populations. ICAM-1 on the surface of epidermal keratinocytes and melanocytes is likely to greatly influence cytotoxic damage of these cells in diseases as photosensitive lupus erythematosus, lichen planus, erythema multiforme, and vitiligo. It has been found that the epidermal staining pattern for ICAM-1 in each of these diseases in distinctive and different in each disease. It is proposed that disease-specific induction of ICAM-1 by factors such as UVR and herpes-virus is an important determinant in triggering these skin diseases and in determining the pattern of disease.