We wanted to study the role of the two TNF receptors (TNFR) in mediating proliferation and nuclear transcription factor kappa-B (NF-kappa B) activation in the human myeloma cell line OH-2. Agonistic antibodies to either of the TNFRs were able to induce proliferation in OH-2 cells, while only antibodies to the p55 TNFR could activate the NF-kappa B. TNF was 100-1000-fold more potent than LT alpha in activation of NF-kappa B and in induction of proliferation in OH-2 cells. Only a 2-fold difference between TNF and LT alpha in affinity for the TNFRs was detected, indicating that the difference in the specific activities of the cytokines can not be explained by different binding affinities. Antagonistic mAbs to either the p55 or p75 TNFR blocked the binding of both cytokines to the cells and significantly inhibited proliferation induced by TNF. On the other hand, only the p55 TNFR mAb was capable of inhibiting the proliferative effect of LT alpha. The p55 mAb 44E potentiated the proliferation induced by LT alpha, but did not affect the TNF-mediated proliferation. The data lead to the following conclusions: (1) both TNFR species trigger proliferation of OH-2 cells, whereas only the p55 TNFR activates the NF-kappa B; (2) TNF signals through both TNFR, whereas LT alpha mediates its signal through the p55 TNFR only; (3) activation of the p55 TNFR by LT alpha is not optimal, but can be facilitated by co-stimulating the receptor with the mAb 44E.