Lymphotoxin (LT) is a cytokine primarily produced by lymphocytes [1]. It is cytostatic or cytotoxic for certain tumor cells in vitro and in vivo. Some cancer cells are particularly sensitive to the cytostatic activity of TNF-α and LT, whereas other cells are insensitive. Combination of TNF-α and interferons (IFNs), particularly IFN-γ, markedly increases the sensitivity in some previously nonresponsive cell lines [2–4]. We investigated the effect of recombinant (r)LT in combination with rIFN-α, hIFN-s, and rIFN-γ on the proliferation of the breast cancer cell line MCF-7.