The 15 kiloDalton major membrane immunogen was included among the Treponema pallidum polypeptides selectively labelled with [3H]-palmitate. The cloned gene for this immunogen, tpp15, encoded a signal peptide of 17 amino acids, a consensus signal peptidase II cleavage site, and a mature protein of 124 amino acids (13,967 Daltons). As predicted by the DNA sequence, the recombinant 15 kiloDalton immunogen labelled selectively with [3H]-palmitate, and globomycin inhibited processing of the precursor to the mature polypeptide. While the native and recombinant immunogens are amphiphilic, the 15 kiloDalton immunogen synthesized in a cell-free system was hydrophilic. The covalent attachment of fatty acids appears to be responsible for the amphiphilicity of the immunogen and its membrane attachment.