In the present study the authors elucidated the involvement of the two TNF receptors (TNFR) in discriminating TNF and lymphotoxin-alpha (LT-alpha) effects in human SW480-beta Gal and KYM-1 cell lines. A non-competitive p55 TNFR monoclonal antibody (MoAb) 44E strongly enhanced LT-alpha-mediated gene regulation and cytotoxicity up to the level of the responses caused by TNF. TNF-induced biological responses were only weakly influenced by 44E. 44E did not affect both binding and the rate of dissociation of the cytokines. The combination of the two p55 TNFR MoAb 44E and Htr5 elicited strong TNF responses, while none of them were agonistic alone. When the p75 TNFR was blocked with Utr1, LT-alpha was still less potent than TNF in mediating CMV promoter activation and cytotoxicity. However, the addition of 44E in the presence of Utr1 merged the LT-alpha dose-response curves with those obtained with TNF plus Utr1. Using antagonistic TNFR MoAb, the authors further showed that TNF functions through both TNFR types while LT-alpha mediates its effects largely via the p55 TNFR. These data suggest that LT-alpha is less potent than TNF due to its lower ability to properly trigger the p55 TNFR and because of its lack of signalling through the p75 TNFR.