Tumor necrosis factor (TNF) has been shown to mediate numerous cellular responses through its interaction with two distinct types of receptor. However, the relationship between TNF receptor and the biological response is not well understood. Modulation of the number of cell surface receptors by various agents has shown a lack of direct correlation with biological responses to the cytokine. In this report, we used several approaches to investigate the relationship between TNF receptor number and an early response in human histiocytic lymphoma U-937 cells. When we examined the activation of the nuclear transcription factor kappa B (NF-kappa B), an event mediated by TNF within 10-15 min, we discovered a correlation between TNF receptor occupancy up to a certain threshold and the extent of activation of the transcription factor. In addition, by kinetically down-regulating TNF receptor expression with phorbol esters, cycloheximide, or trypsin, we determined that receptors were necessary for transduction of the TNF signal. However, 10-25% of total receptors were sufficient for optimum induction of the NF-kappa B signal. When examined in different cell lines, the activation of an early biological response was found to be related not only to the TNF receptor number but also to the type of TNF receptor. These results, overall, suggest that although TNF receptors are essential for induction of NF-kappa B, a small percentage is sufficient to fully transduce this signal.