This paper presents results of three QSAR (Quantitative Structure Activity Relationship) studies realized with the PRECLAV computer program. The database we used contains initially 100 derivatives of 3-carboxy-4-quinolone. The dependent property is bactericidal activity against Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. A specific criterion identifies the outlier molecules in the calibration set. Two molecules are identified as �possible outliers for lead hopping�. After the elimination of outliers, we obtained: N = 77 / 86 / 84, s = 0.2904 / 0.3583 / 0.2993, r2 = 0.8850 / 0.7943 / 0.8645, F = 91.1 / 37.6 / 82.9 and r2CV = 0.8415 / 0.7337 / 0.8415. The bactericidal activity against the three studied bacteria was favored by the presence of saturated C substituted (hetero)cycles, by the presence of certain groups (-F, unconjugated -NH/-NH2) and by a non-balanced molecular shape. The bactericidal activity was disfavored by the presence of certain chemical groups (-NO2, -C6H4, -CO-) and of the triazole cycle. The lipophilic/hydrophilic feature of quinolones has little impact upon bactericidal activity.