Functionalized polymers have gained much interest in the last decades. This is due to their functional group and their polymer nature that give them unique properties and more advantages than the corresponding small molecules. In this trend, polyhydroxystyrene-co-MMA was modified to introduce amino group in the side chain of the polymer. The amine modified polymer was reacted with two classes of active compounds. The first class is aldehydes such as vanilline, p-hydroxybenzaldehyde, p-chlorobenzaldehyde, and anisaldehyde. The second class is phenolic esters such as p-hydroxymethyl-benzoate, 2,4-dihydroxymethyl benzoate, and methyl salicylate. The antimicrobial activities of the polymer and modified polymer with these two classes were explored with Gram-negative bacteria (Escherichia coli), Gram-positive bacteria (Bacillus subtilus), fugus like yeast (Candida albicans SC5314), and pathogenic molds (Aspergillus flavus, Trycophyton rubrum, and F. oxysporium). In vitro studies indicated that the start polymer did not affect on the test microorganisms, in contrary to its derivatives. The diameter of inhibition zone varied according to the active group in the modified polymer, polymer microstructure, and the test microorganism. Derivatives I, II, and III were selected among the most effective antimicrobial compounds. Their inhibitory effects on the ratio of surviving cell number (M/C) increased by increasing derivatives concentrations. Derivatives I and II were inhibitorier to C. albicans and molds than to bacteria while derivative III was only antibacterial. These derivatives seemed toxic to Brine shrimp by increasing their concentrations above 10 ppm, with derivative III being the less toxic, compared to others. To clarify this toxic effect and to decrease the toxicity of these derivatives, more detailed studies are necessary, and this will be focused in the nearest future. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2734-2742, 2011