The extraction of plant constituents is essential to isolate biologically active compounds and understanding their role in disease prevention, treatment and in knowing their toxic effects as well. However, meager information is available about the properties and biological activities of phytochemicals derived from certain plants found in Allahabad and adjoining areas. Keeping this information in view, we prepared aqueous extracts and determined their biochemical properties including their impact on the activity of human RBC's acetylcholinesterase (AChE). The UV—Visible spectrophotometric profiles of the aqueous extracts of different parts of the four plant species viz. Calotropis procera, Datura metal, Cannabis sativa, Argemone mexicana and Thevitia peruviana displayed two major peaks at 302 and 336 nm corresponding to the presence of different flavonoids in these preparations. These extracts indicated presence of protein in the range of 1.12 to 19.25mg/g wet weight of the plant tissues. The impact of different phytochemicals present in these extracts was studied on the activity of AChE isolated from human erythrocytes (RBCs). The extracts derived from Argemone mexicana and Datura metal exhibited strong AChE inhibitory potential, whereas others did not show significant inhibition even at higher concentrations. The results indicate that human RBC's can be used as a potential biomarker towards evaluation of the efficacy and toxic potential of varied plant extracts.