Nalidixic acid (NA) and piromidic acid (PA) increase the rate of electron transfer from FeSO4 to cytochrome c in in vitro experiments under aerobic conditions. Several compounds related to PA showed similar effects which correlated with their antibacterial activity against Gram-positive bacteria. Two new compounds of 1, 10-phenanthroline structure were synthesized and shown to have a more potent antibacterial activity with a stronger acceleratory effect than that of NA and PA. Absorption spectral analysis showed that acceleratory effects of NA and PA were due to chelate formation between Fe2+ and a mediator as well as intermediate formation during the electron transfer from Fe2+ to cytochrome c.