This study evaluated the bacterial toxicity of lanthanum oxide micron and nano sized particles using shake flask method against gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria. Particle size, morphology and chemical composition were determined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Re-sults indicated that lanthanum oxide nanoparticles showed antimicrobial activity against Staphylococcus aureus, but not against Escherichia coli and Pseudomonas aeruginosa. It was speculated that lanthanum oxide produced this effect by interacting with the gram-positive bacte-rial cell wall. Furthermore, lanthanum oxide bulk particles were found to enhance the pyocyanin pigment production in Pseudomonas aeruginosa.) This study evaluated as the bacterial toxicity of lanthanum oxide micron and nano sized particles using shake flask method against gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria. Particle size, morphology and chemical composition were determined using scanning Re-sults indicated that lanthanum oxide nanoparticles showed antimicrobial activity against Staphylococcus aureus, but not against Escherichia coli and Pseudomonas aeruginosa. It was speculated that lanthanum oxide produced this effect by interacting with the gram-positive bacte-rial cell wall. Further, lanthanum oxide bulk particles were found to enhance the pyocyanin pigment production in Pseudomonas aeruginosa.)