The continuous mink lung epithelial cell line Mv1Lu has proven to be a sensitive reporter line in the bioassay for purified TGF-β, exhibiting a sigmoid-shaped concentration-response relationship with an EC₅₀ of 12 pM (0.3 ng/mL). Maximal inhibition of Mv1Lu cells generates a 75–95% decrement in the number of adherent cells. However, this bioassay is not specific for TGF-β as originally claimed. Mv1Lu cells are sensitive to other cytokines and substances found in complex biological fluids. In this study the effects of other biological response modifiers in this assay were tested and several were found to have important growth modulatory capacities that confound the quantitation of TGF-β- EGF, TGF-α, fibronectin, and IGF-I all induce Mv1Lu cell proliferation. In contrast, neither PDGF (-AA, -AB, -BB) nor endotoxin (≤ 10 ng/mL) affect Mv1Lu cell number. TGF-β and TNF-α at high concentrations (≥ 10 ng/mL) are the only cytokines examined that inhibit Mv1Lu proliferation. TGF-β decreases final cell number both by preventing mitosis and by inhibition of adherence of cells to the un-coated dish. Several strategies are suggested to assure the specificity of this otherwise convenient bioassay for TGF-β.