Abstract: The sample (10 µL) was treated with aqueous 0.3 mM Fe(III) solution (90 µL) and acetate buffer (200 µL) and the mixture was incubated at 30°C. Proteins were pptd. by addition of aqueous 1.25 M HClO4 (200 µL). After centrifuging, the supernatant solution was analyzed by flow injection analysis with use of aqueous 0.5 M NaSCN as the detection reagent; the complex formed was monitored at 480 nm. Tests carried out on human and rabbit serum revealed the presence of a ferroxidase-like substance other than ceruloplasmin. The coefficient of variation (n = 6) was 3.4%. No interferences were observed in the presence of Zn(II), Ni(II), Cd(II), Pb(II), Ca(II), Mg(II), Cl-, PO43-, SO42-, CO32-, CO32-, ClO4-, NO2- or S2O3-, whereas Cu(II), Cr(III), N3-, NO3- and SCN- interfered.