OBJECTIVES: Irisin is a myokine with 112 aminoacids and its level is regulated by PGC1-α.It is released into blood circulation from skeletal muscle tissue after a proteolytic cleavage of extracellular domain of FNDC5.Aprotinin is a polivalent serin protease inhibitor.It is added to the sample solutions such as serum, plasma or tissue extracts in order to inhibit the serine proteases found in the sample medium.So, degradation of the proteins to be measured can be prevented.These study has been made to get a preliminary information whether it is neccessary to add aprotinin in serum and plasma samples to prevent any irisin loss in samples which are needed to be kept at -80°C for a long time.MATERIALS and METHODS: For this purpose, blood samples have been taken from 10 men and 10 women volunteers with ages between 25-40 and aprotinin has been added to the plasma and the serum samples and have been kept at -80°C for 3 months.At the end of 3 months, irisin levels of these samples with aprotinin and without aprotinin have been determined by ELISA.RESULTS: Statistical analysis of the results has shown an insignificant differance between the plasma samples with or without aprotinin (p=0.525) and a significant decrease between the serum samples with and without aprotinin (p=0.009).CONCLUSIONS: In conclusion, with the results of this study, no net decision could have been achieved to add aprotinin to the samples for irisin determination with ELISA in plasma and serum kept at -80°C for about 3 months.