As is true for other peptide hormones, TNF causes the down-modulation of its own receptor. The process by which down-regulation occurs and the particular role of each of two recently identified receptors, however, are not understood. In this report we used Abs specific to p60 and p80 TNFR types to examine the ligand-induced down-regulation in histiocytic cell line U-937. These cells express both types of TNFRs, but the amount of p80 is two to three times greater than that of the p60 receptor. Treatment of U-937 cells with the ligand led to maximum down-modulation of TNFR within 30 min, and this decrease was found to be a result of receptor number and not affinity. When examined for the receptor type, approximately 90% of the p60 receptor and 35% of the p80 receptor was down-regulated by the cytokine. Rapid internalization of TNF in U-937 cells is mediated through p60, because this process was inhibited only by anti-p60 Ab and was also inhibited in cells (PMA-pretreated) that primarily express the p80 receptor. In contrast to p60, we observed that the ligand-induced shedding of the p80 receptor into the medium results in the down-regulation. Interestingly, however, the signal for shedding of the p80 receptor appears to be mediated through the p60 receptor, because anti-p60 Ab inhibited the shedding. Overall, our results provide evidence that ligand-induced down-modulation of TNFR is a result of the internalization of p60 and of the shedding of the p80 receptor and that the signaling for both is mediated through the p60 receptor.