The peroxidase activity of human hemoglobin (Hb) was inhibited more than 90% above 1.5 mM hydroxylamide ($NH_2OH$) within 1 min at pH 6.0. The inhibition by hydroxylamine wsa reversible, and showed the biphasic inhibition pattern in which the peroxidase activity was reduced initially and then formed a plateau region regardless of hydroxylamine concentrations. The analysis of double reciprocal plot and Dixon plot at different concentrations of hydroxylamine suggested that hydroxylamine was a noncompetitive inhibitor, and $K_i$ value was $270;{mu}M$. The time course of spectral changes, obtained after the incubation of $7.2;{mu}M$ human hemoglobin with 50 mM hydroxylamine or $20;{mu}M$ hemin with 100 mM hydroxylamine, indicated that hydroxylamine decreased the absorption maxima of 403 nm region of human hemoglobin or hemin. The radical scavengers, such as mannitol or dimethylsulfoxide (DMSO), did not reduce the inhibition effect of hydroxylamine. It was proposed that hydroxylamine inhibited the peroxidase activity of human hemoglobin by binding noncovalently to the prosthetic heme groups.