The copper resistance in Escherichia coli determined by plasmid pRJ1004 is inducible. The level of resistance is proportional to the inducing dose of copper. The level of copper resistance in induced and uninduced cells changes with the growth phase of the culture. Induced resistant cells accumulate less copper than uninduced cells, so that reduced accumulation may be the mechanism of resistance. We propose that the inducible plasmid-coded copper resistance interacts with the normal metabolism of the cell to protect against toxic levels of copper while allowing continued operation of copper-dependent functions.