In Neisseria gonorrhoeae, all penicillinase-specifying plasmids isolated so far share homology with each other and carry a 60% deleted sequence of TnA. Plasmid pHD131, an element isolated from Haemophilus ducreyi and carrying an intact copy of the ampicillin resistance transposable element, was introduced from Escherichia coli into N. gonorrhoeae by both transformation and conjugative mobilization. Plasmids were recovered with no detectable deletion. After their transfer back into E. coli, transposition assays onto phage-lambda DNA demonstrated that the TnA elements were still functional. Plasmid pHD131 could be stably maintained in N. gonorrhoeae with or without the presence of penicillin. These results support the hypothesis that the absence in N. gonorrhoeae of plasmids carrying entire and functional TnA transposons cannot be ascribed to incompatibility between the genetic element and the host, but rather to a barrier to introduction of foreign DNA into gonococcus.