The dimer catastrophe hypothesis has been proposed previously to explain instability of multicopy plasmids whose partitioning is random, contrary to low copy number plasmids which are stably maintained and actively partitioned. Until now, this hypothesis has been investigated using multicopy ColE1 plasmids. However, for more detailed testing of the dimer/multimer catastrophe hypothesis, one should use a plasmid which can be maintained at either low or high copy number and still possesses the same mechanism of replication regulation. Here we used a modified lambda plasmid, pTC lambda 1. The advantage of this plasmid is that it can be maintained at different copy numbers depending on the concentration of an inducer which stimulates the initiation of plasmid replication. Results obtained with this plasmid in recombination proficient and deficient cells generally support the dimer/multimer catastrophe hypothesis, but also suggest some modification in the model.