The role of basic fibroblast growth factor (bFGF) in the control of teratoma cell proliferation was examined. It was found that bFGF stimulates proliferation at low concentrations but induces cell migration at higher doses. These effects could be efficiently counteracted by addition of protamine sulphate. Moreover the bFGF gene is actively transcribed in primary human testicular embryonal carcinomas but was not expressed in any other embryonic tumour examined. The biological implications of these findings will be discussed.