Tumor necrosis factor-alpha (TNF-alpha) and lymphotoxin (LT, TNF-beta) are pleiotropic cytokines involved in diverse biologic processes, including immune and inflammatory reactions. The biologic responses to TNF are mediated through two forms of cell surface receptors, p55R and p75R. Both receptors exist in a soluble form (p55-sR or TBP I and p75-sR or TBP II), generated by the proteolytic cleavage of the extracellular regions of the molecule. These soluble forms may act by binding and, hence, neutralizing circulating TNF. In the present study, the murine A9 cell line in vitro bioassay was used to test TBP I and TBP II for their neutralizing activity against recombinant human TNF-alpha (rHu-TNF-alpha), and TNF-beta (rHu-TNF-beta) and recombinant murine TNF-alpha (rMu-TNF-alpha). Moreover, TBP I and TBP II were tested for their ability to displace TBP I in the TNF-TBP receptor binding assay (RIBA) against human and murine TNF-alpha as well as TNF-beta. TBP I, from either recombinant (from CHO and Escherichia coli) or urinary origin, was the most effective inhibitor with respect to rHu-TBP II (from CHO) against either human or murine TNF-alpha both in the A9 cells bioassay and in the RIBA assay. Both TBP I and TBP II preparations were less effective in protecting the A9 cells from the toxic effects of rMu-TNF-alpha than from those of rHu-TNF-alpha. The rHu-TBP II preparation was the most effective in inhibiting the cytocidal effect of rHu-TNF-beta on A9 cells and as active as TBP I in the RIBA assay. This result seems to indicate rHu-TBP II as the better soluble TNF receptor able to reverse the rHu-TNF-beta-induced toxicity, at least on A9 cells, leading to consideration of its therapeutic use in those diseases, such as multiple sclerosis, where a role for TNF-beta is indicated.