Bovine colostrum contains transforming growth factor (TGF)-beta-like activity. High levels of this activity are found in early colostrum (within 12 h after parturition); however, it decreases rapidly after 30 h. In this study, the activity in early colostrum was purified by a sequence of decaseination, DEAE-Sephacel chromatography, Sephadex G-100 gel filtration, precipitation at neutral pH, further gel filtration and reversed-phase FPLC. A homodimeric 23 kDa protein was identified that retained the TGF-beta-like activity. Its activity was completely neutralized by a specific anti-TGF-beta 2 antibody. These results indicate that the TGF-beta-like growth factor found in bovine colostrum is chemically and antigenically related to TGF-beta 2. The physiological roles of the TGF-beta 2-related growth factor are discussed.