A series of 94 benzoic acid derivatives was synthesized and tested for its ability to inhibit influenza neuraminidase. The enzyme-inhibitor complex structure was determined by X-ray crystallographic analysis for compounds which inhibited the enzyme. The most potent compound tested in vitro, 5 (4-(acetylamino)-3-guanidinobenzoic acid), had an IC50= 2.5× 10-6 M against N9 neuraminidase. Compound 5 was oriented in the active site of the ...