The blood level of C-reactive protein and erythrocyte sedimentation rate reflect inflammation and are useful for the diagnosis of bacterial infection. However, these markers are often increased in other diseases such as rheumatoid arthritis. Procalcitonin (PCT), a precursor of calcitonin, was reported to be produced at the time of bacterial infection. The detection of PCT in blood is especially useful for the diagnosis of bacteremia. PCT is also considered to be useful for the diagnosis of limited bacterial infections, such as pneumonia, meningitis, and pyelonephritis, although the level in these conditions could be much less than that in bacteremia. There are two methods for the measurement of PCT in Japan: the chemiluminescence enzyme immunoassay (CLEIA) and immunochromatography assay (IC). CLEIA is quantitative and is sensitive for detecting a low level of PCT. IC is semi-quantitative and is useful for bed-side testing. It is important to understand the features of these two methods of PCT and to use them in appropriate situations.