AbstractGenomic clones encoding bovine TGF-αwere identified by hybridization with probes derived from human TGF-αsequence. Nucleotide sequence of the clones predicts that mature bovine TGF-αis a 50 amino acid polypeptide which shares 96% and 92% homology with human and rat TGF-α, respectively. Bovine TGF-αwith the predicted sequence was chemically synthesized and tested for activity. Synthetic bovine TGF-αcompetes in a radioreceptor assay with labelled mouse EGF with activity parallel to that of human TGF-αand mouse EGF. The mitogenic activity of bovine TGF-αis comparable to that of human EGF in causing proliferation of bovine mammary epithelial cells. An∼5.0 kilobase RNA transcript is observed in polyadenylated RNA from MDBK cells by Northern blot analysis. The polymerase chain reaction detects the presence of a TGF-αtranscript in many bovine tissues. These data indicate that bovine TGF-αmay be a normal regulator of cell growth in the bovine animal.