Acidic and basic fibroblast growth factors (FGF) were identified in bovine cheese whey after partial purification using a two step procedure. Cation-exchange chromatography produced a mitogen-rich extract which was loaded on to a heparin—sepharose column and eluted stepwise with 0.8, 1.2 and 2.0 m-nh 4 HCO3. Mitogenic activity was found in all three fractions by cell growth assays using Balb/c-3T3 fibroblasts. Immunoblotting identified acidic FGF in the 1–2 M-eluate and basic FGF in the 2.0 M-eluate, but neither acidic nor basic FGF was detected in the 0.8 M-fraction. Quantitative radioreceptor assays indicated 5.8 ng of acidic FGF-like activity and 19.8 ng of basic FGF-like activity perl whey in the appropriate eluates. This study represents the first direct demonstration of FGF in milk.