Human serum paraoxonase 1 (PON1) is an Ca-dependant enzyme that is synthesised in liver and in blood is mainly associated with a high density lipoprotein (HDL). PON1 has organophosphatasa, arylesterase and lactonase activity and it hydrolyzes many different substrates. Because of its antiatherogenic function and antioxidant activity there is an increasing interest in clinical and epidemiological studies. Arylesterase activity of PON1 (ARE) is determined by using the substrate phenylacetate, mostly from serum samples. The aim of the study was to determine if a second sample except serum can be suitable for determining the ARE. The plasma samples were used with anticoagulants: EDTA, citrate and heparin. The study showed that a sample of EDTA plasma is not good for determining ARE because EDTA binds calcium, which is important for PON1 activity. In a sample of citrated plasma PON1 enzymatic activity is reduced as compared to the serum, but not statistically significant, and such a sample can be used to determine the ARE. The most reliable sample other than serum, is one with heparin as an anticoagulant because it gives the minimum deviation value of the enzymatic activity PON1 as compared to the serum. Although PON1 is stable up to 6 months by storing the sample at -20 °C, it was found that storage of the sample serum, EDTA, citrate and heparin plasma at + 4 °C PON1 enzyme activity values vary. The activity of the PON1 fifth day in the EDTA plasma sample is reduced by more than 90%, and in a sample of serum, citrated and heparin plasma by 10% in relation to the first day. The results of the investigation show that the enzymatic activity of PON1 is preferably measured immediately after the sampling.