To evaluate the efficacy of an antimicrobial ceramic for killing Legionella strains in vitro, bacteria were exposed to the ceramic soaked in PBS at 25 degrees C or 42 degrees C. The number of L. pneumophila began to decrease significantly after 4 h of exposure at 25 degrees C and reached < 10 log cfu/ml after 12 h. A similar significant decrease was also observed after exposure at 42 degrees C. Furthermore, it was found that the antimicrobial ceramic showed bactericidal activity against six strains of Legionella isolated from various water sources, including L. pneumophila (serotype 1-4), L. micdadei, and L. dumoffii, after 24 h of exposure. The antimicrobial activity against L. pneumophila of the supernatant obtained by soaking the ceramic in PBS for 24 h was also assessed. Bactericidal activity of this supernatant was also noted. Analysis of the supernatant by ICP-MS resulted in the detection of eight metals (Mg, Al, Ca, Mn, Zn, Sr, Ag, and Ba) at a maximum concentration of 2.5 mg/l. When reconstituted PBS was made with all eight metals at the same concentrations as in the supernatant, the reconstituted PBS containing Ag alone and all metals showed significantly bactericidal activity against L. pneumophila, but PBS with only one metal component except Ag or a combination of Ag with Zn and/or Ca did not. These findings suggest that the antimicrobial ceramic possesses strong bactericidal activity against Legionella species and that eight metals released from the ceramic have a synergistic bactericidal effect against Legionella. When the antimicrobial ceramic was placed in hot spring water or cooling tower water instead of PBS, the number of L. pneumophila in the water decreased to < 10 log cfu/ml after 24 h of exposure and the bactericidal activity persisted for 5 weeks. These results indicate that the antimicrobial ceramic can be used to eradicate Legionella species contaminating various water sources.