The concentration of EGF immunoreactivity in rat whey increases from 0.3 pmol/ml at lactation day 1 to 2.0 pmol/ml at lactation day 19. The concentration of EGF is not influenced when the rats undergo sialoadenectomy prior to mating. On S-200 gel chromatography, almost all EGF-reactivity in rat whey elutes as a broad peak corresponding to a Stokes radius of 4.0 nm (an approximate molecular weight of 80 kDa). Almost no 6 kDa EGF is present. Judged by gel filtration of whey pre-incubated with 125I-EGF (6 kDa), no binding protein for EGF is present in rat whey. When rat milk is incubated overnight at 37 degrees C, the 80 kDa EGF is degraded and elutes as a peak with a Stokes radius of 2.7 nm, corresponding to a molecular weight of approximately 35 kDa EGF and as a peak corresponding to 6 kDa EGF. Also, after partial purification by immuno-affinity chromatography, the EGF-reactive material in rat whey behaves as a peptide with a Stokes radius of 2.7 nm, corresponding to a molecular weight of approximately 35 kDa at gel filtration. Comparative binding studies between EGF purified from the submandibular glands and the EGF purified from rat whey confirm differences in the binding to antibodies raised against submandibular EGF, but not in binding to the EGF-receptor. Our results make it unlikely that EGF in rat whey is derived from the submandibular glands.