On day 10 of a primary infection of Trichinella spiralis in NIH mice, when adult worms were in the process of being expelled, their fecundity was markedly depressed. Upon transplantation to new host mice, the worms were able to establish successfully in their usual position in the anterior small intestine, even if originally recovered from the large intestine of mice expelling their worms, and these worms fully recovered their fecundity after 7 days in recipients. On day 5 of secondary and tertiary infections, immediately before loss of worms began, the adult worms were stunted and had a depressed fecundity. However, such worms, when transplanted, were able to establish in normal numbers and in a normal position in the intestine of recipient mice. Additionally, these worms were found to have developed a fecundity, and grown to a length, comparable to that of normal worms, but slightly less than that of control worms transplanted from day 7 of a primary infection, after 7 days in a new host. Electron microscopy showed that worms taken at the time of expulsion in primary and tertiary infections showed structural abnormalities, including large quantities of lipid in intestinal cells and somatic musculature and decreased quantities of stored glycogen. Upon transplantation to new host mice, these worms were found to have recovered a normal ultrastructural appearance within 7 days, although there was still evidence of a slightly elevated lipid and depressed glycogen content. It is concluded that immune expulsion of T. spiralis from mice does not involve irreversible damage to the worms.