The efficacy of ampicillin bound to polyisohexylcyanoacrylate nanoparticles was studied in vitro in mouse peritoneal macrophages infected with Listeria monocytogenes. Nanoparticles containing ampicillin 1 mg/L were more effective after 30 h than free ampicillin at the same concentration, with viable counts of 3.68 and 5.43 log10 cfu/mL, respectively. The nanoparticles acted on the intracellular bacteria after a lag period of 6-9 h; this time was apparently required for the degradation of the polymer. At the doses used in these experiments, empty nanoparticles had neither an anti-listeria nor a cytotoxic effect.