The principal product from the cyclization of [3-(5-nitropyridine-2-yl)-amino]pyridine-2-yl}thioaceticacid is 2-[3-nitropyridine-2-yl)amino]pyridine-2-yl}thioacetyl-8-[(5-nitropyridine-2-yl)amino]-3-oxidopyrido[2,1-b]thiazolium (4). Its structure was determined primarily by 1H and 13C nmr. [Journal translation]