An attempt was undertaken to determine whether a concurrent infection of Trichinella spiralis and T. pseudospiralis can reduce cellular infiltrations against the former species during the muscle phase of worm development. BALB/c, nude and CBA/N mice were orally infected with either species or a mixture of both. New-born larvae (NBL) of either or both species were also injected subcutaneously into the right/left leg of BALB/c mice. In T. spiralis oral infection, myositis was strongest in BALB/c, intermediate in CBA/N and weakest in nude mice. In T. pseudospiralis oral infection, slight cellular infiltrations were observed around the worms in BALB/c but not in nude or CBA/N mice. However, in mixed oral infections of two species, infiltrations around the sites of T. spiralis were not reduced. In mice injected with T. pseudospiralis NBL, infiltrations around the infective-stage larvae were mostly absent. However, in mice injected with T. spiralis NBL, prominent granulomatous reactions were observed near the sites of worms. The tissue reaction was substantially stronger than that in oral infections. In mice injected with NBL of both species (into different legs), a heavy infiltration was also observed at the site of T. spiralis. A marked increase in levels of IL-4 and IL-6 was detected in the popliteal lymphocytes of BALB/c mice injected with either live or dead NBL of T. spiralis at days 15 and 20 post-injection. This indicated that the worms mainly elicited a TH2 response during the muscle phase of development. An indirect fluorescent antibody test and laser confocal microscopic studies demonstrated the presence of CD4 and CD8 cells in the cytoplasmic region of the nurse cell complex of T. spiralis.