We previously reported that the Enhanced Ultrasonography (EU) with carbon dioxide (CO2) microbubbles was useful for the diagnosis of hepatic tumors. Recently, we applied helium gas (He) microbubbles as a new contrast agent for EU. Before the clinical challenge, we examined the acute toxicity for the organs using a experimental dog. No apparent histological change of its liver and lung was found before and after the injection of the He microbubbles into the hepatic artery. The contrast enhancement of hepatic tumors by He-EU was just as effective as that by CO2-EU. The effect of enhancement of hepatocellular carcinomas (HCC) by He-EU continued longer than that by CO2-EU, 12.8±4.18 min., and 3.5±1.62 min., respectively (pMoreover, after the disappearance of contrast enhancement of the non-tumorous liver parenchyma, hepatic tumors were still clearly observed for over 30 min. After the complete enhancement of the total hepatic parenchyma by the He microbubbles, the back flow of contrast agents to the splenic artery was observed. This phenomenon suggested that the He microbubbles filled the hepatic artery and blocked its blood flow. The He microbubbles, used with some anticancer agents, may be expected to have some chemoembolization effects on hepatic tumors.