An integrated flow-injection processing (FIP) system for the quantification of plasmids during cultivation is described. The system performs on-line sampling, cell lysis, and quantification of plasmids in an integrated manner during cultivation of E. coli. The system was operated by using a miniaturized expanded-bed column which can be used for handling samples containing cells and cell debris without interfering with the binding analysis. Two types of detectors (one measuring UV absorbance at 254 nm and a fluorometer) are used for online plasmid detection. The system was developed using standard solutions and it was successfully applied in monitoring plasmid contents during a cultivation of E. coli.