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 x 10(-6) M against N9 neuraminidase. Compound 5 was oriented in the active site of the neuraminidase in a manner that was not predicted from the reported active site binding of GANA (4) with neuraminidase. In a mouse model of influenza, 5 did not protect the mice from weight loss due to the influenza virus when dosed intranasally.