Summary: A highly purified dextran of large molecular weight has been found to be antigenic in guinea pigs. Delayed dermal reactions and PCA antibody were produced upon immunization with this polysaccharide in Freund's complete adjuvant. The incidence of actively acquired immunity to this antigen is highest among guinea pigs of the Abyssinian strain. The majority of Pirbright, Hartley and family XIII guinea pigs do not become immunized to dextran. The ability of guinea pigs to actively develop immune responses to dextran has been tentatively interpreted as genetically determined. Adoptive transfer of the delayed dermal reactivity for dextran was accomplished with lymphoid cells but not serum. Transfers were successful from sensitive Abyssinian animals to normal Abyssinian animals (responders) but not to Pirbright animals (nonresponders). The inability of Pirbright animals to demonstrate delayed dermal responses to dextran following adoptive transfer of competent lymphoid cells has been viewed as a separate, second defect. The latter has been termed an inability of “translation.”