Serum and plasma calprotectin concentration is shown to be elevated when neutrophils are activated, and may therefore be used as a marker for inflammatory diseases. A serum calprotectin immunoassay was developed based on calprotectin values observed in samples from the intensive care unit. The polyclonal avian antibodies were raised and affinity purified with calprotectin antigens. The performance was tested and it was observed that the assay was linear in the range 0.3-24.7 mg/L, the limit of quantitation was observed to be lower than 0.3 mg/L, no antigen excess was observed up to 54 mg/L, all CVs were lower than 1.8 % in the precision study, the calibration curve stability was longer than 6 weeks, and there was no significant interference detected for haemoglobin, intralipid or bilirubin. The serum calprotectin immunoassay presented in this paper performs well within the criteria carefully set from the limited clinical experience obtained in both serum and plasma. In addition it is commutable with Bühlmann MRP8/14 ELISA.