Bis(5-aminofluorescein)dichloroplatinum(II), and substituted fluoresceinamine derivatives of this compound are disclosed as highly effective radiation sensitizers which enhance the effect of ionizing radiation on tumor cells, but uniquely do so without the usual accompanying high toxicity level to surrounding normal cells. A direct combination, single step reaction synthesis is disclosed for preparing bis(5-aminofluorescein)dichloroplatinum(II) from the reaction between an alkali metal tetrachloroplatinate(II) and 5-aminofluorescein
