We report that IL 1 acts on the endothelium, inducing a long-lasting increase in its adhesivity to tumor cells. Selective pretreatment of cultured human umbilical vein endothelial cells (EC) with IL 1 caused a significant increase in adhesion of three human colorectal carcinoma (HT-29, HCC-P2988, and HCC-M1410) cell lines and one human melanoma (A-375) cell line. Tumor necrosis factor (TNF) was as effective as IL 1 in promoting tumor cell adhesion to EC, whereas IFN gamma and IL 2 were inactive. The IL 1 and TNF induction of EC adhesivity was both concentration (threshold concentration 1 U/ml) and time dependent (peak 4-6 h), reversible within 24 h, and blocked by a protein synthesis inhibitor. The IL 1 and TNF action on EC may play a role in tumor cell lodgement.