Studies of biological properties of newborn larvae of Trichinella spiralis and immune responses against newborn larvae in the host are reviewed. The biological properties of newborn larvae examined are the natural production, migration, surface antigens and their maturity and effects on the host. Various mechanisms of host immunity to newborn larvae are reviewed, including the stage specificity, kinetics, cell-mediated vs. antibody-dependent immunity in the mucosal and other tissues of the host.