Tumor necrosis factor α (TNFα) induces the apoptotic death of primary villous cytotrophoblasts in culture (Yui et al., Placenta 1994; 15:819). Since both p55 and p75 TNF receptors (TNFRs) localize to the villous trophoblast, we examined their roles in mediating trophoblast apoptosis. Comparison of ¹²⁵I-TNF α binding competition by receptor-specific antibodies revealed 2.7-fold more TNFRp75 than TNFRp55. Immunohistochemical analysis of receptor distribution showed TNFRp75 to be expressed strongly in < 20% of cells and TNFRp55 moderately in approximately 50%. Culture with TNFα increased the percentage of cells expressing TNFRp75 to > 40% but had little effect on TNFRp55 expression. Agonistic anti-TNFRp55 antibody and TNFRp55-specific TNF mutant protein stimulated both apoptosis and loss of trophoblast viability. In contrast, TNFRp75-specific mutant TNFα protein failed to induce either of these responses. Furthermore, neither cell death nor apoptosis stimulated by wild-type TNFα was inhibited by an antagonistic anti-TNFRp75 antibody. Thus, the apoptotic death of primary cytotrophoblasts is mediated almost entirely by TNFRp55, and the p75 receptor appears to have little effect on the process.