Randomly bred guinea pigs of both sexes were injected intracardially with one-half a 50% lethal dose of Listeria monocytogenes. When these animals were skin tested with 30 ,g of a water-soluble extract of sonically disrupted Listeria, animals had uniformly detectable levels of delayed-type hypersensitivity (DTH) 6 days after infection. Histological examination of skin test reaction sites, after fixation in Helly fixative and Giemsa staining, revealed a classical tuberculintype infiltrate consisting primarily of mononuclear cells with few polymorphonuclear cells. Many of the small vessels showed perivascular cuffing. When purified peritoneal exudate lymphocytes from these animals were cultured in vitro in the presence of various concentrations of Listeria antigen, it was found that the optimal antigenic dose for specific antigen-induced incorporation of [3H Ithymi-
MATERIALS AND METHODSAnimals. Randomly bred albino guinea pigs of both sexes were obtained from local suppliers. The 647 on July 15, 2020 by guest http://iai.asm.org/ Downloaded from
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