The ribosomal protein L7/L12 isolated from Brucella melitensis induces a delayed-type hypersensitivity (DTH) reaction in brucella-sensitized guinea pigs. Surprisingly, the recombinant brucella L7/L12 protein expressed in Escherichia coli as a fusion protein with a six-histidine tag cannot elicit such a reaction. The six histidines tagged to the recombinant L7/L12 protein were removed enzymatically, but the resulting protein did not induce a DTH reaction in sensitized animals. Incubation of the recombinant L7/L12 fusion protein in a B. melitensis lysate endowed the recombinant protein with a DTH activity, suggesting that the recombinant protein was modified by this treatment. Glycosylation or phosphorylation of the recombinant L7/L12 protein could not be detected. On the other hand, radiolabeled palmitic acid was found to be incorporated to the recombinant protein during its incubation in the brucella lysate. This incorporation was specific for the brucella L7/L12 protein and was inhibited when the brucella lysate was frozen and thawed before the incubation. The data reported here indicate that posttranslational modification of L7/L12 protein comprising at least an acylation step is required for the brucella L7/L12 DTH activity.