Abstract—Phenylheptatriyne (PHT) plus near‐ultraviolet light(320–400 nm; NUV) hemolyzed human erythrocytes in an oxygen dependent manner. When the phototoxicity of PHT plus NUV was tested with a series ofEscherichia colistrains carrying all four possible combinations of genes controlling excision proficiency (uvrA6 vs uvrA+) and catalase activity (HPII,katF vs katF*), the membrane was found to be an important lethal target. Consistent with this observation. PHT plus NUV did not induce histidine independent (his‐4+) mutations in the four tester strains (RT7h‐RT10h). Using tester strain RT10h, it was shown that there was no inactivation by PHT plus NUV in nitrogen. Results of experiments with anE. colifatty acid auxotroph (K1060) treated with PHT plus NUV are also consistent with membrane proteins being the chief targets for attack. Radicals were formed during the photolysis of PHT plus NUV in aqueous solutions, both in the presence of air and under nitrogen. Since PHT plus NUV did not hemolyze erythrocytes or inactivateE. colicells under nitrogen, these radicals are not