The Hepatitis B virus surface antigen gene was cloned into a yeast plasmid vector with a complete 2-micron origin and an ars origin, and the resultant recombinant plasmid, pMYN101, was transformed into several yeast strains, AH22, AH22, cir-0, A281, and LL20 cir-0. The recombinant plasmid was found to be very stable in all of these strains tested suggesting that the phenomenon of plasmid incompatibility due to the coexistence with another 2-micron plasmid does not occur. For comparison a few other kinds of yeast plasmids with an incomplete 2-micron origin or an ars origin were transformed into the same yeast hosts and their stability was studied in a batchwise operation. These plasmids showed an unstable yet interesting behaviour making careful prediction of plasmid stability based upon the complete characteristics of both the plasmids and the hosts, and their interaction an important necessity.