Studies on the immunity to Trichinella spiralis in mice are reported. In three experiments, the worms of a single immunizing infection were either eradicated or sexually sterilized prior to the deposition of larvae. Immunity was induced in the mice, as indicated by a 54 to 69% reduction in the enteral phase of a challenge infection, and a 53 to 91% reduction in the parenteral phase. In a fourth experiment mice were exposed to 6 abbreviated immunizing infections (each of 1 day''s duration) before being challenged. Although the control challenge infection was light and variable, the immunized mice showed a reduction of > 99% in the parenteral phase of the challenge infection. In the final experiment, one group of infected mice was so treated as to allow the enteral phase to run its course, but without the production of larvae. In another group, the adult worms were allowed to produce larvae until the 21st day of infection. An attempt was then made to eradicate the infection in both groups, and the mice were subsequently challenged. The immunity acquired by the mice with the enteral-only infection was at least as strong as that acquired by the mice with the biphasic enteral-parenteral infection. This was in spite of the fact that the latter mice harbored a number of "immunizing" larvae at the time of challenge, whereas the former did not. While the failure of the biphasic infection to induce a stronger immunity may have been due to the debilitating nature of this infection and its intensive therapy, the results support the conclusion that the parenteral phase of trichinosis does not enhance the immunity induced by the enteral phase.