Background and Aims: Adenosine deaminase (EC 3.5.4.4, ADA) is distributed in all human tissues. It catalyses the deamination of adenosine (deoxyadenosine) to inosine (deoxyinosine) via removal of an amino group. The determination of ADAinhibition is of extreme significance in the field of fundamental research and clinical diagnosis. In our study, the effects of various plant extracts and chemicals on ADA activity were investigated. Methods: Adenosine deaminase activity was determined spectrophotometrically at 625 nm. Results: The inhibitory activities of the extracts and chemical compounds increased in a dose-dependent manner. Among the plant extracts used, lemon extract was observed to exhibit the highest ADA inhibition activity with an IC 50 value of 0.05 ± 0.001 mg/mL, while quercetin had the highest ADA inhibition (IC 50 value of 0.004±0.0005 mg/mL) among the chemical compounds. All plant extracts and chemical compounds showed ADA inhibition activities. Conclusion:The obtained results indicate that plant extracts and some chemical compounds examined in this study can be a potential source of novel ADA-inhibitors for therapeutics.