Alpha-fetoprotein (AFP) is a transporter of various serum ligands and regulator of cellular growth during pregnancy. Estrogens modify AFP to exhibit growth suppressive properties. We recently synthesized a peptide (P149) from human AFP that suppresses the growth of mouse uterus and MCF-7 breast cancer cells. Here it is shown that molar excess treatment of native AFP with estradiol-17 beta (E2) exposes the P149 site on AFP. The anti-estrogenic and anti-tumor activities of AFP-peptides were tested in vivo in the immature mouse uterine assay and mammary tumor (6WI-101)-induced ascites assay, and in vitro in a cytostatic assay using five different human breast tumor cell lines. AFP-peptide P149, and fragments of P149, P149A and P149C but not P149B, suppressed the growth in both in vivo assays. P149 also suppressed the in vitro growth of MCF-7, MDA-MB-231, MDA-MB435 breast cancer cells by more than 75%. P149 and P149A bound the estrogen receptor-alpha (ER) with low affinities compared to E2 and tamoxifen, while P149B bound 3H-E2 with 10(5) fold less affinity compared to ER. The recent epidemiologic observation that high AFP levels in young pregnant women reduce their subsequent risk of postmenopausal breast cancer may be related to the growth suppressive property of AFP with the exposed P149 epitope.