Mice infected with Trichinella spiralis have shown resistance to both intravenous and intraperitoneal infection with Listeria monocytogenes. Since protection against L. monocytogenes is mediated by cellular immunity, the effect of infection with T. spiralis on the immune response to BCG was investigated. ICR/CD1 mice were infected with 200 T. spiralis at 14, 56, 98 and 112 days preceding intravenous administration of 4 × 106 viable BCG. Groups were tested for delayed hypersensitive foot pad responses 28 days following BCG inoculation. All mice receiving T. spiralis and BCG exhibited a marked potentiation of the 24-hour foot pad response to Old Tuberculin over that shown by the appropriate controls.