We isolated a transposon (Tn4291) that carries the resistance gene(s) for methicillin in a secondary insertion site on the penicillinase plasmid pI524. Transposition of Tn4291 into pI524 occurred during the transduction of the tetracycline resistance plasmid pSN1 from a methicillin-resistant donor into a recipient that carried the mec allele in the primary site on the chromosome. Insertion of Tn4291 caused extensive rearrangement of pI524 and resulted in the formation of a 27.9-kilobase-pair plasmid (pIT103) which coded for resistance to methicillin and cadmium, but not penicillin. Although resistance to methicillin and cadmium were always linked, Tn4291 was stably maintained only in the presence of a chromosomal mec allele, while in its absence the plasmid was unstable and transposition to the primary site occurred. Subsequently, a 20.1-kilobase-pair plasmid, pIT203, was formed which retained cadmium resistance and regained the ability to express beta-lactamase activity.