The cell dynamics of the receptor on tumor necrosis factor (TNF) were studied with the use of TNF-sensitive KYM cells derived from human myosarcoma. With receptor synthesis inhibited by cycloheximide, the half-life of the surface TNF receptor was 2h in the absence of TNF and 30min in its presence, suggesting that the TNF receptor was non-recycling and that its internalization was accelerated by TNF. During cell incubation with suppression of TNF receptor degradation by chloroquine, the number of surface TNF receptors remained approximately constants, but the total number of surface and internal TNF receptors increased gradually, at 3h reaching 1.5 times of the initial number, thus suggesting continuous synthesis, externalization, internalization, and degradation of the TNF receptor in the absence of cycloheximide. When the cells were incubated with 125I-TNF, the intracellular quantity of the pulse-labeled TNF-receptor complex promptly increased, reaching a maximum at 20 min, and then declining gradually. Thus, it was confirmed that the TNF receptor is internalized as a TNF-receptor complex in the presence of TNF. In incubation with suppression of protein synthesis by cycloheximide following surface TNF receptor digestion by trypsin, TNF receptors reappeared on the cell surface, increasing in the number to a peak level at 60 min and gradually decreasing. The cells previously exposed to cycloheximide with or without TNF showed no recurrence of surface TNF receptors, suggesting that the TNF receptor is non-recycling. The results thus suggest that the TNF receptor is continuously internalized and degraded intracellularly by lysosomes without being recycled regardless of the presence or absence of TNF, and further that its internalization is accelerated when it is part of the TNF-receptor complex.