Sensitized lymphoid cells could transfer to normal non-sensitized (naive) mice by 24-48 h after antigen challenge in the ear, the capacity to incorporate, at the site of antigen deposition, 5-iodo-2′deoxyuridine-125I in amounts significantly above those obtained in control mice. This was associated with a mononuclear cell infiltration in the pinna. In contrast to lymphoid cells, serum antibodies were unable to transfer a 24- or 48 hour ear reaction. The cells responsible were T lymphocytes as demonstrated by successful transfer following enrichment for T lymphocytes, and abrogation of transfer following treatment with anti-Θ serum and complement. Transfer was achieved whether the naive recipients were normal, T-cell deprived, pretreated with cyclophosphamide, or lightly irradiated but not when they were heavily irradiated. Adoptive transfer of the 24-hour ear response was demonstrated with three different antigenic systems. The time-response curves were different with each system although peak reactions were obtained 5 days after sensitization of the donors in all cases. The specificity patterns of the 24-hour ear reactions on transfer were similar to those obtained in the sensitized donors. The results of these studies indicate that the radioisotopic ear method can, under defined conditions, demonstrate the existence of a state of delayed type hypersensitivity in the donors.