A number of transition metal complexes, including the cis and trans isomers of dichlorodiammineplatinum (II), six complexes of rhodium (I), two of iridium (I), and one of ruthenium (II) have been tested for their ability to induce lambda prophage, to produce filamentous growth of Escherichia coli , and to be selectively toxic for strains with defects in the deoxyribonucleic acid repair system. Dichlorotetrakis(dimethylsulfoxide)ruthenium II [RuCl 2 (DMSO) 4 ] was strictly similar to cis -dichlorodiammineplatinum II [ cis PtCl 2 (NH 3 ) 2 ] in the test for lambda induction, filamentous growth production, and selective toxicity for a recA − strain. [Rh COD 1,10-phenanthroline] + Cl − , though more toxic for recA − than for rec + E. coli , was scarcely effective in the test for filamentous growth and did not induce prophage. None of the other tested compounds showed any similarity with cis -PtCl 2 (NH 3 ) 2 . Due to the interesting results obtained with cis -PtCl 2 (NH 3 ) 2 as an antitumor agent, it seems reasonable to propose RuCl 2 (DMSO) 4 as a potential antitumor substance.