This work presents the evaluation of plasmid stability in E. coli BL21(DE3) along high cell density cultures in bioreactors. Two distinct constructions of plasmids were used: plasmid pJ404, with the Native protein gene and the plasmid pJ411, with the Mutated protein gene. The retention of plasmids, even in high cell concentration, until 50 g/L was quite satisfactory, was around 90-100% of both plasmids. Despite results are preliminary, they indicate that, at least for the both plasmid evaluated, even in high cell density cultures, E. coli BL21(DE3) keeps plasmids to produce the recombinant proteins.