Carrageenan containing a trace of polypeptide induced, without adjuvant, an intense and persistent delayed hypersensitivity in guinea pigs. This hypersensitivity was transferred by peritoneal exudate cells, and the delayed reaction was not associated with demonstrable Arthus-type skin reaction after a single-dose immunization. The roles of the polysaccharide component and of the polypeptide portion of the carrageenan in induction and elicitation are discussed.