The Transplant Acceptance-inducing Cell (TAIC) is a form of immunoregulatory macrophage which appears to be capable of establishing a state of alloantigen-specific partial tolerance of solid organ transplants in renal transplant recipients. This report establishes that human TAICs do not elicit a proliferative response from co-cultured allogeneic T cells. More significantly, it has been shown that TAICs are able to suppress proliferation of allogeneic CD4(+) and CD8(+) T cells after mitogenic stimulation. This suppressive activity can be attributed to the deletion of activated T cells by TAICs and to the secretion of a soluble inhibitor of T cell proliferation. It is further shown that the acquisition of T cell-suppressive properties by human TAICs depends upon an unidentified component of human serum.