Sensitization of B10.A mice with oxazolone results in delayed-type hypersensitivity (DTH) and a two to three-fold increase in the weight of regional lymph nodes by the third day. Hydrocortisone treatment of the animals 3 days after sensitization suppresses the manifestation of DTH on the tenth day but not on the seventieth day and significantly decreases the weight of lymph nodes both in control and oxazolone-sensitized mice. The regional lymph node cells of oxazolone-stimulated mice were, like the controls, sensitive to dexamethasone in vitro, as judged by viability and thymidine incorporation. Dexamethasone binding capacity of the regional lymph node cells 72 h after oxazolone sensitization increased. However, the dissociation constant for dexamethasone, the binding specificity, and the nuclear transfer of the hormone were not altered. Since high doses of hydrocortisone given 3 days after sensitization were unable to erase permanently the DTH to oxazolone, it seems very likely that cells responsible for the immunological memory survive the high dose of hydrocortisone.