If bovine serum albumin (BSA) is covalently conjugated with dodecanoic acid, its ability to stimulate delayed-type hypersensitivity (DTH) is enhanced whereas its ability to stimulate antibody production is suppressed. Conjugates of BSA prepared with negatively charged succinyl groups and positively charged amidine groups were unable to stimulate any detectable DTH to BSA even though their ability to bind anti-BSA antibody in a radioimmunoassay was similar to that of dodecanoyl-BSA which stimulated strong, sustained DTH. By analyzing a series of conjugates of BSA with fatty acids of varying chain length, we found that enhancement of the ability of a preparation to stimulate DTH to BSA could be separated from the suppression of its ability to stimulate antibody formation. The ability of a preparation to stimulate DTH correlated with the covalent conjugation of a large number of hydrophobic groups whereas the suppression of the ability to stimulate antibody formation correlated with covering of antibody-combining sites as measured in a radioimmunoassay.