This report provides evidence to support the hypothesis that tumor necrosis factor-alpha (TNF-alpha) and IL-4 promote the expression of new endothelial surface molecules in rejecting murine heterotopic cardiac allografts. The microvascular endothelia of these cardiac allografts all develop strong reactivity with the monoclonal antibodies (mAbs) YN1.1/74 (anti-ICAM-1), M/K-2 (anti-VCAM-1) and MECA-32 (undefined molecule) within 3 to 5 days of graft implantation. Daily treatment of the allograft recipients with pentoxifylline (PTX), soluble TNF receptor (TNFR:Fc), anti-interleukin-4 (IL-4) mAb (11B11), or soluble IL-4 receptor, each abrogate the expression of endothelial VCAM-1 and reduce the endothelial reactivity with the mAbs YN1.1/74 and MECA-32 to levels found in cardiac isografts. This is accompanied by a reduction, but not an elimination, of interstitial leukocytic infiltration. Despite this, cardiac allograft recipients treated with PTX or the mAb 11B11 rejected allografts at the same rate as untreated allograft recipients, ie, within 10 to 12 days after graft implantation. These rejected grafts contained mRNAs for TNF-alpha and IL-4, as well as for all other cytokines that have been associated with rejecting allografts. This suggests that endothelial VCAM-1 expression, which is characteristic of rejection, is not an essential element of the rejection process. Interestingly, the grafts from the PTX-treated recipients continued to display rare, isolated VCAM-1 positive cells in the interstitium, which may be dendritic cells. In general, these studies demonstrate a role for IL-4 and TNF-alpha in the alterations of vascular endothelial phenotype observed in rejecting cardiac allografts. They also demonstrate that endothelial VCAM-1 expression is not essential for the allograft rejection process, and that the role of VCAM-1 in this process may be more subtle than was initially suspected.