Mice born of mothers that had been sensitized to Trichinella spiralis by 2 infections with living larvae or by antigen-adjuvant injections were immune to a T. spiralis challenge when 3 weeks old. This immunity waned in the young mice by the 6th week of life. Experiments suggested that most of the immunity was passed from the mother to the young through the milk, with only minor placental passage indicated. Serologic tests demonstrated agglutinating trichinella antibodies in the sera collected from infected mothers, mice born and nursed by infected mothers, and mice born of normal mothers and fostered by infected mothers. Passive cutaneous anaphylaxis (PCA) tests were negative with these sera. The results indicated that the immunity in the baby mice was passively acquired from the sensitized mother and not actively produced by the young. The hypothesis of an antibody-mediated Arthus-type hypersensitivity is suggested. Several workers have observed the transfer of immunity to Trichinella spiralis from immune mothers to their offspring. Mauss (1940) demonstrated that the offspring of T. spiralisinfected rats, rabbits, and hamsters were less susceptible to infection with trichinella than were the offspring of uninfected controls. Culbertson (1943) showed that the transmission of immunity in rats was mediated through the milk suckled by the offspring. Little or no transmission of immunity through the placenta was thought to occur. Duckett et al. (1972) recently reported that mice born of uninfected mothers acquired immunity to trichinella when fostered by trichinella-immune mothers, but when the litters from immune mothers were fostered by uninfected mothers, no immune expulsion of adult trichinella was observed. The authors suggested that antibodies in the mothers' milk may be responsible for the passive immunity in the suckling mice. The present report, using Swiss mice from this Department, describes four experiments designed to determine whether immunity to trichinella is passed from mothers to their offspring. This strain of mouse has repeatedly manifested delayed (cellular) hypersensitivity to the intestinal phase of trichinella; however, Received for publication 20 August 1973. * Department of Medical Zoology, Walter Reed Army Institute of Research, Washington, D. C. 20012. many attempts to transfer immunity with hyperimmune sera have failed (Larsh, 1970). MATERIALS AND METHODS Swiss white mice were used for all experiments during this study. This strain has been maintained in this Department as a randomly outbred colony since 1943. The strain of T. spiralis was isolated originally from a pig in 1936. It was maintained in laboratory rats for the first 7 years after isolation, and since has been maintained in the abovedescribed Swiss mice. Larvae used for infection were obtained from mice infected approximately 3 months. The technique used for collecting and standardizing the infective inoculum was described by Larsh and Kent (1949) and later modified by Weatherly (1970). The methods for recovering and counting adult worms after a challenging infection were those used by Larsh and Kent (1949). The methods used for recovering and counting larvae were originally described by Larsh and Kent (1949). T. spiralis larvae used for antigen preparation were collected from infected mice by the technique described previously. The collected larvae were washed in commercially prepared physiologic soluion (TIS-U-SOL; Travenol Laboratories, Inc.). After 2 washings, the larvae were refrigerated (4 C) overnight in TIS-U-SOL (about 5.5 ml of TIS-U-SOL to 0.5 ml packed larvae). After this period, the antigen was prepared from these larvae as described by Larsh et al. (1969). Blood for serologic tests was collected from mice by orbital bleeding. Polyethylene tubing with an inside diameter of 0.047 and an outside diameter of 0.067 inch was cut into 3-inch lengths. One en of the tube was inserted into the orbital sinus, and the blood was collected in 10by 75-mm plastic tubes. The tubes were kept at room tem-