Norfloxacin complexes Co(nor)_2·8H_2O(nor=norfloxacin) and [Co(nor)(phen)]NO_3·2H_2O(phen=1,10-phenanthroline) were synthesized and their biological activity on Tetrahymena and Escherichia coli(E.coli) was assayed by means of microcalorimetry. Massive experimental parameters such as the growth constant k, inhibitory ratio I, half inhibition concentration IC_(50) and generation time TG were obtained. Data showed that with the increase of exposing complex concentration, both complexes exhibited strong inhibition during the growth of organisms. While toxic degrees were quite different on Tetrahymena and E.coli, due to various toxic mechanisms. And complex molecular volume and the ability of the complex penetrating into cells may be the keys. Norfloxacin complexes Co (nor) _2.8H_2O (nor = norfloxacin) and [Co (nor) (phen)] NO_3 · 2H_2O (phen = 1,10-phenanthroline) were synthesized and their biological activity on Tetrahymena and Escherichia coli Massive experimental parameters such as the growth constant k, inhibitory ratio I, half inhibition concentration IC 50 (50) and generation time TG were obtained. Data showed that with the increase of exposing complex concentration, both complexes DRAW strong inhibition during the growth of organisms. While toxic degrees were quite different on Tetrahymena and E. coli, due to various toxic mechanisms. And complex molecular volume and the ability of the complex penetrating into cells may be the keys.