The tuberculin activity of ribosomal material isolated from BCG was studied. Ribosomal material was able to induce delayed-type hypersensitivity reactions in BCG-sensitized guinea pigs, in addition to early inflammatory reactions in both BCG-sensitized and non-sensitized animals. Ribosomal RNA and protein were isolated from the above material and their tuberculin activity was studied. Whereas purified RNA was unable to elicit any inflammatory reaction, ribosomal protein did induce a delayed-type hypersensitivity reaction in BCG-sensitized guinea pigs. No early inflammatory reactions were observed with the latter material. In addition, ribosomal protein sensitized guinea pigs not only against the specific ribosomal protein but also against PPD. Its activity was destroyed by pronase treatment but resisted heat treatment.