Promethazine has been recognised as an effective antiplasmid agent in cultures containing a single bacterial species such as Escherichia coli, Yersinia enterocolitica, Staphylococcus aureus and Agrobacterium tumefaciens. The objective of this study was to examine the effect of heterogenity of the microbial flora on plasmid elimination by promethazine in a laboratory based model system of mixed bacterial infection. F'lac plasmid elimination of E. coli K12 LE140 was studied in the presence of a numerically predominant Gram-positive species (Bacillus cereus, Staphylococcus epidermidis) by promethazine (0-120 mg/l) at 23, 37 and 39 degrees C. Growth kinetics of different bacterial species were studied at various temperatures without drug treatment in mixed bacterial cultures and it was found that the small number of added bacteria overgrew the pre-existing flora during the incubation period. We observed that bacterial-bacterial interactions modified the growth rate of individual bacterial species and gave selective advantages to some bacterial species of the microbial community. Some interactions between coexisting bacterial species enhanced the frequency of plasmid curing by promethazine in mixed cultures with S. epidermidis. In our experiments the plasmid curing action of promethazine was more effective at elevated temperature than at lower temperatures.