A comparative study of the mutagenic activity of 21 derivatives of biphenyl was performed in the strain TA1538 of Salmonella typhimurium. The position effect of carboxyl, amide, aldehyde, and ether groups was examined. The para position of substituents, among which at least one is a nitro group, causes mutagenic activity in most of the molecules studied. Derivatives of biphenyl that have no substituents in the para position were inactive. In addition, 4-nitrobiphenyl (4-NBP), 4-nitro-2'-carboxy biphenyl (4-N-2'-C-BP), 4,4'-dinitro-2'-carboxy biphenyl (4-4'-DN-2'-C-BP), and 4,4'-dinitro-2,2'-carboxy biphenyl (4-4'-DN-2,2'-DC-BP) induced no frameshift mutations in TA1538. The most active was 2,4,4'-TN-6-C-6'-Ad-BP, giving up to 800 revertants per nmol; 2,4,6,2'-TN-4'-6'-DC-BP and 2,4,2',4'-TN-2'-C-BP, which induced 250 and 350 revertants per nmol, respectively, were highly active frameshift mutagens.