In immunized mice, penetration of the liver by 3H-thymidine or 3H-amino acid-labeled FV3 inoculated by the intraperitoneal route was inhibited, and the virus was blocked in the peritoneal cavity and in the intestine. The efficiency of such a peritoneal blockage has been demonstrated in non-immunized animals pretreated intraperitoneally with thorotrast. However, this is not sufficient to explain the protection observed after a large challenge dose of virus. Absence of penetration into the hepatic cells has been shown to be another mechanism involved in the protection of immunized mice.