4-Chlorosalicylic acid (4-Chloro-2-hydroxybenzoic acid, 4-chloro salicylic acid) was used in sensitive spectrofluorometric determination of terbium in mixed rare earths and preparation of poly(4-chlorosalicylic acid-formaldehyde) via condensation with formaldehyde. 4-Chlorosalicylic acid is a pharmaceutical intermediate. Inhibits monophenolase and diphenolase activity with IC50s of 1.89 mM and 1.10 mM. Potent antimicrobial activity. Against E. coli with the MIC of 250 μg/mL and with the MBC of 500 μg/mL.