Hybrid plasmids were constructed by combining in vitro the Escherichia coli plasmid pGA22, which carries the genes determining resistance to kanamycin, tetracycline, chloramphenicol and ampicillin, with the cryptic plasmids, pCG1 and pCG2, of Corynebacterium glutamicum. The hybrid plasmids were introduced into C. glutamicum and E. coli and replicated in both hosts. They expressed all the E. coli resistance phenotypes except ampicillin resistance in C. glutamicum. The levels of antibiotic inactivating enzymes encoded on these plasmids were about four to ten times lower in C. glutamicum than in E. coli. Despite the lack of expression of ampicillin resistance, beta-lactamase activity was detected in C. glutamicum carrying hybrid plasmids.