KYM-1D4 cells are a subline derived from a human rhabdomyosarcoma which are highly sensitive to TNF-mediated cytotoxicity. They were selected for this study because they express human TNF-R and are therefore a more relevant target for comparing the potential therapeutic value of human TNF-inhibitory agents than the usual murine cell lines. Two recombinant soluble TNF-R-IgG fusion proteins, one containing p55 TNR-R, the other containing p75 TNF-R, and a recombinant monomeric soluble p55 TNF-R were all found to block the cytotoxicity generated by human TNF-alpha and LT as well as also murine TNF. The p55 TNF-R-IgG fusion protein (p55-sf2) was the most effective of the antagonists tested, requiring an equimolar, (based on a monomeric configuration of TNF-alpha) or a 3-fold higher (based on a trimeric configuration of TNF-alpha) molar concentration to inhibit the cytotoxicity mediated TNF-alpha by 50%. p55-sf2 was also as effective at inhibiting the cytotoxicity mediated by LT or murine TNF in the KYM-1D4 assay. In contrast, the monomeric soluble p55 TNF-R was the least effective inhibitor, requiring a > 4000-fold higher molar concentration than p55-sf2 to achieve a similar degree of protection. The fusion proteins, particularly p55-sf2, may be useful as human therapeutic agents, as at low concentrations they can prevent both TNF-alpha-mediated and LT-mediated effects on human cells. As TNF-R-IgG fusion proteins also block the action of murine TNF in vitro, they may also be useful in the investigation of murine models of human inflammatory disease.