In studying plasmid instability in recombinant microorganisms, Escherichia coli MV 12[p VH5] and MV 12 trpR[p VH5] harboring trp operon were used as experimental model systems. The host with the trp repression system was partially derepressed by 3‐beta‐indoleacrylic acid. The results from kinetic analysis of plasmid instability showed that the stability of p VH5 and the growth rate of MV 12[p VH5] decreased rapidly in presence of 3‐beta‐indoleacrylic acid at a concentration higher than 10 μg/mL, but beyond 30 g/mL no significant change was observed. This suggests that Trp⁻ variants from MV 12[p VH5] could be produced from the host cells at a different frequency depending on the physiological condition. In another system, MV 12 trpR[p VH5] which was constructed by conjugation of E. coli MV 12[p VH5] with E. coli CSH61, the plasmid stability was much lower and the frequency of Trp⁻ cell production was about 10 times higher as compared with the MV 12[p VH5] when treated with 3‐beta‐indoleacrylic acid. A kinetic model representing the plasmid instability was derived, and a fairly good agreement with the experimental results was found. The fraction of plasmid‐free cell (or negative variant) shows a different time course profile depending on the segregation coefficient (a, production rate of negative variants from positive cells during one generation), growth ratio (G, the ratio of growth rates of negative variants to positive cells), and other parameters. The negative variant fraction continues to increase (“run away” type) or it approaches to a finite value (“settling” type) depending on the relative values of a and G.

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