WF-2421 is a novel aldose reductase inhibitor produced by Humicola grisea. WF-2421 was purified from the culture filtrate by successive ion exchange chromatography and the chemical structure was assigned to be alpha-formamido-5'-(2-formamido-1-hydroxyethyl)- beta,2',6-trihydroxy-3-biphenylpropanoic acid (1) on the basis of spectroscopic evidence. The IC50 value of WF-2421 was 3 x 10(-8) M against partially purified aldose reductase of rabbit lens.