In this research, in vitro serine protease inhibitory activity of 10 plant species was evaluated, and extracts that showed strong activity were analyzed through high-performance liquid chromatography (HPLC). Rhododendron caucasicum Pall. (leaf) and Potentilla reptans L. were found to have the highest chymotrypsin inhibitory activities (83.77% and 82.01% inhibition). The highest trypsin inhibitory activity was observed in R. caucasicum (flower) (82.86% inhibition), followed by Cruciata laevipes Opiz (82.22% inhibition). Extracts showing potent enzyme inhibition were fractioned and subjected to activity tests. The highest chymotrypsin inhibitory activity was observed in the n-hexane fraction of P. reptans (92.90% inhibition). In comparison, the highest trypsin inhibitory activity was found in the ethyl acetate fraction of Lythrum salicaria L. (89.81% inhibition). HPLC studies determined that the 80% ethanol extract of P. reptans contained chlorogenic acid. The screened plants were generally rich in phenol and flavonoid content and showed strong antioxidant activity.