An integrated flow injection process for analysis of intracellular components of microbes has been used to monitor plasmid content in Escherichia coli cultivations inoculated with cells subcultured in the presence or absence of ampicillin. The system allows sampling, sample handling, cell disruption, separation of intracellular components, and analysis in a semi-on-line mode of operation. The time scale for the assay is in the range 15 min (plasmid peak) to 25 min (complete assay cycle). As expected, lower initial plasmid content was found using an inoculum subcultured in the absence of ampicillin. More importantly, significant decrease in plasmid content was detected in the later stages of the cultivations (grown in ampicillin containing medium) even when using inoculum subcultured in the presence of ampicillin. This illustrates the versatility of the system, which allows monitoring of plasmid content as the cultivation proceeds.