A flow cytometric quantitative analysis of receptors on neutrophils can be exploited in distinguishing between inflammatory and infectious diseases. In this prospective comparative study, simultaneous quantitative analysis of CD64 and CD35 on peripheral blood neutrophils was performed in febrile patients in order to differentiate between bacterial infections (n=89), viral infections (n=46), and inflammatory diseases (n=21). The patient data was compared to 60 healthy controls. We could divide patients into three groups depending on how they express CD35 and CD64 on neutrophils: (1) patients with a high probability of viral infection (low CD35/low CD64 and low CD35/high CD64), (2) patients with a high probability of inflammatory disease (high CD35/low CD64), and (3) patients with a high probability of bacterial infection (high CD35/high CD64). In summary, simultaneous quantitative analysis of CD64 and CD35 on neutrophils could potentially assist physicians to distinguish between inflammatory and infectious diseases.