The purpose of this study is to develop an assay system for quantification of bFGF in human tissue and to investigate the changes in bFGF content in the human placenta during pregnancy. Sixty-two placental tissue samples from various stages of normal pregnancies were collected. Approximately 28 micrograms bFGF was obtained per 1 kg of placental tissue. The recovery rates were 17.1 +/- 7.4%. The purified samples were confirmed as bFGF by SDS-PAGE and enzyme-linked immunoelectrotransfer blot (EITB) with anti-human bFGF monoclonal antibody. The bFGF readings in the human placenta determined by RIA were 11.81 +/- 2.11 fmol/mg protein (first trimester), 20.45 +/- 4.85 (early second trimester), 9.52 +/- 5.02 (late second trimester), 7.41 +/- 2.07 (third trimester), and 7.75 +/- 1.86 (post trimester). The placental bFGF were significantly high in the early stage of second trimester and declined gradually during the remainder of the pregnancy. The RIA values were correlated closely with the values obtained by bioassay. These results demonstrate that our assay system provides a tool for the quantification of bFGF in biological samples and suggest that bFGF, the active mitogen and angiogenic factor, participates in the formation of the human placenta.