Deproteinization is a necessary step in the measurement of serum NOx (nitrite+nitrate) concentrations. This study aims at comparing different protein precipitation methods for measurement of serum nitric oxide end products concentrations by the Griess method. For this purpose ten protein precipitation methods which most of them previously used for deproteinization in serum NOx determination applied for protein removal. Serum samples from healthy volunteer deproteinaized with different methods, subsequently serum NOx concentrations were determined by the Griess assay. Pearson correlation coefficients and mean differences between values determined for each method (using ultrafiltration as reference method). P-values less than 0.05 were considered significant. Nitrite standard curve had linearity up to 150 µmol/l (r²=0.998). Our results showed that acid solutions are not suitable for protein removal in serum NOx determination. Using methanol, ethanol, and diethylether/methanol resulted with higher serum NOx values. Acetonitrile and zinc sulfate had good agreement with ultrafiltration method for serum NOx determination  in terms of their mean differences from ultrafiltration method (-1.6 ± 8.6 and -2.3 ± 7.4  respectively). It can be concluded that zinc sulfate and acetonitrile are the methods of choice for protein removal in determining serum NOx concentrations by the Griess method.