Experiments on the P 388 D1 cell line (48 h exposure) demonstrate that [1,2-bis-(fluorophenyl)ethylenediamine]platinum(II) complexes are comparably active on the cell number and 3H-thymidine incorporation, irrespective of the position of the fluorine atom (ortho, meta, or para) and the nature of the "leaving group" (Cl- or H2O). However, the compounds of the R,R/S,S series are more active than those of the R,S series and comparable to cisplatin. In the "tumor colony forming assay" the R,R/S,S configurated compounds are about ten times as active as cisplatin. The R,R/S,S configurated diaqua[1,2-bis(4-fluorophenyl)ethylenediamine]platinum(II) salts reach their half maximum effect more readily (t1/2 approximately equal to 1.6 h) than their R,S configurated analogues (t1/2 approximately equal to 20 h). A time limited contact of the cells with R,R/S,S configurated diaqua[1,2-bis(4-fluorophenyl)ethylenediamine]platinum(II) salts (-1h) leads to a similar inhibition like a permanent drug exposure indicating a fast uptake of the complex by the tumor cell. In experiments on the Ehrlich ascites tumor of the mouse and on the L 1210 leukemia cell line R,R/S,S-[1,2-bis(4-fluorophenyl)ethylenediamine]dichloroplatinum(II) turns out to be equipotent with cisplatin.