3,4-Dihydroxybenzoic acid is reported as dietery polyphenol. Antiviral activity of 3,4-dihydroxybenzoic acid (protocatechuic acid, PCA) against a virulent H9N2 strain in a mouse model is reported. Cytotoxic effects of PCA on 3 non-small cell lung cancer (NSCLC) cell lines, A549, H3255, and Calu-6 cell lines is reported. PCA is reported to prevent the carcinogenesis or antitumor growth in vivo. Signaling pathway involved in PCA-induced apoptosis in human gastric adenocarcinoma (AGS) cells is reported. The oxidation of 3,4-dihydroxybenzoic acid by H2O2 in aqueous/goethite slurry at varying operating conditions (catalyst load, temperature, pH, substrate and hydrogen peroxide starting concentration) is investigated.