Transforming growth factor-β (TGF-β) and insulin-like growth factor (IGF-I) can attenuate drug-induced cell death in epithelial cells. Since milk whey contains a mixture of these and other growth factors, we evaluated mitogenic bovine whey extract (MBWE) for protective activity against chemotherapy drug damage in cultured epithelial cells (mink lung, Mv1Lu). Etoposide and vinblastine reduced cell survival by up to 90%. This was attenuated by the addition of MBWE before and during drug exposure, but not following drug removal. MBWE was compared with individual growth factors known to be present in the mixture. IGF-I and platelet-derived growth factor were ineffective, whereas TGF-β2 induced growth inhibition and cell survival, with a maximum response at 3 ng/ml. TGF-β2 bioactivity was also demonstrated by showing that acidification of MBWE (A-MBWE), to activate TGF-β2, enhanced its growth inhibitory and chemoprotective activities 60- and 12-fold, respectively. However, MBWE contained additional protective factors. When TGF-β2 and the MBWE preparations were compared, on the basis of growth inhibition equivalents, MBWE protected cells against drug toxicity at concentrations an order of magnitude lower than with TGF-β2 or A-MBWE. Immunoneutralization of the TGF-β present in MBWE and A-MBWE eliminated all growth inhibitory activity but not all cell survival activity. We conclude that the MBWE preparations are cytoprotective against two chemotherapy drugs when added before and during drug exposure. TGF-β contributes to this activity, but the extracts contain other factors that promote the survival of epithelial cells after chemotherapy drug exposure.