The effect of plasmid introduction into Lactococcus lactis subsp. lactis IL2661 on the growth of this strain and on plasmid stability was studied in pure batch cultures. The plasmids used (coding for erythromycin or chloramphenicol resistance) were the following: pIL205 (42 kb), pIL252 (4.6 kb, 6–9 copies), pIL253 (4.8 kb, 45–85 copies) and pE194 (inserted in the chromosome). Growth and acidification of L. lactis subsp. lactis IL2661 were similar to those of the derived recombinant lactococci. The maximal population at the end of the fermentation (9 h) was about 1.1±0.3 × 1010 cfu/ml, and maximal growth rate 0.92±0.07 h–1. Growth yield and lactic acid concentrations were 3.9±0.8 × 1011 cfu/g lactose consumed and 25.6±2.3 g/l, respectively. Different levels of plasmid stability were detected. Plasmid pE194, and plasmids pIL252 and pIL253 in the absence of pIL205, were stable after 10 h of culture. A slight loss (1–2%) of pIL205 was observed in all strains. In the presence of pIL205, plasmids pIL252 and pIL253 were maintained in only 56–95% of the cells. This result suggested an incompatibility between pIL205 and pIL252 or pIL253.