Alpha-fetoprotein (AFP) is a major serum protein produced during fetal development. Experimental findings suggest that AFP has antiestrotrophic activity and that it can be developed as a therapeutic agent to treat existing estrogen-dependent breast cancer or to prevent premalignant foci from developing into breast cancer. The antiestrotrophic activity of AFP was reported to be localized to a peptide consisting of amino acids 447-480, a 34-mer peptide termed P447. A series of parsings and substitutions of amino acids in the P447 sequence was intended to identify the shortest analog which retained antiestrotrophic activity. Peptides related to P447 were generated using solid phase peptide synthesis. Several shorter peptides, including an 8-mer called P472-2 (amino acids 472-479, peptide sequence EMTPVNPG), retained activity, whereas peptides shorter than eight amino acid residues were inactive. The dose-related antiestrotrophic activity of AFP-derived peptides was determined in an immature mouse uterine growth assay that measures their ability to inhibit estradiol-stimulated uterine growth. In this assay, the maximal inhibitory activities exhibited by peptide P472-2 (49%), by peptide P447 (45%), and by intact AFP (35-45%) were comparable. The octapeptide P472-2 was also active against estradiol-stimulated growth of T47D human breast cancer cells in culture. These data suggest that peptide P472-2 is the minimal sequence in AFP, which retains the antiestrotrophic activity found with the full-length molecule. The synthetic nature and defined structure of this 8-mer peptide suggest that it can be developed into a new drug which opposes the action of estrogen, perhaps including the promotional effects of estradiol in the development of human breast cancer.