Multiple injections of bovine serum albumin (BSA) without adjuvant stimulate production of large amounts of antibody but no detectable delayed type hypersensitivity (DTH) in guinea pigs. In contrast, single or multiple injections of BSA heavily conjugated with a lipid, dodecanoic acid, without adjuvant, stimulated sustained DTH but no detectable antibody production. The DTH produced was specific for native BSA and no detectable immune response was produced against the fatty acid groups. This change in the type of immune response produced was associated with a change in the area of antigen localization in popliteal lymph nodes. Native BSA labeled with 125I was localized in germinal centers in the bone marrow derived cell area but was not detectable in the thymic dependent or thymus derived (T) cell area of the lymph nodes. In contrast, lipid conjugated BSA labeled with 125I was prominently localized in the thymic dependent area of the lymph nodes but was not found in the germinal centers of animals producing pure DTH reactions. These results suggest that the well known ability of lipids to enhance the production of DTH may be in part due to a lipid-mediated localization of antigen within the thymic dependent area of lymph nodes where it is in close association with many T type lymphocytes.