Objectives: C-reactive protein (CRP) reflects measure of response acute phase. We aimed to assess prozone phenomenon that causes false negativity in serological diag‌nosis of CRP. Materials and methods: In this study the tests of four cases of 600 participants that have false negative serological results were evaluated by agglutination and turbidometry methods. Findings: CRP test was positive in titer of 1/8 in all cases but the first case remains positive to titer of 1/16.the results of turbidometry method demonstrated be positivity all samples. Conclusion: By regarded to other laboratory findings, in the laboratories that used agglutination method, samples that suggested high CRP in order to exclude false sero-negativity must be diluted to upper titers.