During physiological stimulation of resting T-cells, at least two activation signals by antigen presenting cells are required. Besides the first antigen-specific signal, the second costimulatory signal involves CD80 and CD86 expressed by the antigen presenting cell. These costimulatory molecules have been suggested to be of clinical relevance in many different autoimmune and malignant disease processes. We previously observed that tumor cells in Kaposi's sarcoma (a common AIDS-related cutaneous neoplasm) completely lack both CD80 and CD86, and these tumor cells fail to stimulate T-cell proliferation. In this study, using a Kaposi's sarcoma tumor cell line designated SLK, various stable transfected cell lines were produced. Tumor cells that were either singly positive for either CD80 or CD86, as well as a double-positive cell line, were examined for their ability to induce T-cell activation, T-cell proliferation, and cytokine production profiles. Compared to the parental double-negative tumor cell line, the CD80-positive cells, but not the CD86-positive tumor cells, induced significant T-cell activation and proliferation. Tumor cells expressing both CD80 and CD86 also induced T-cell activation. After stimulation by the transfected tumor cells, T-cells produced a Th-1 type cytokine production profile with increased IL-2 and IFN-gamma levels. These results demonstrate that Kaposi's sarcoma tumor cells lacking co-stimulatory molecules cannot induce T-cell activation; however, if they express CD80, they can induce peripheral blood T-cell proliferation, and there is a differential response as expression of CD86 did not have the same immunostimulatory effect.