Insertion of the transposable genetic element Tn 1 into different sites of plasmid ColE1 results in a number of mutant phenotypes. Whereas all plasmids examined were present in normal amount, all showed reduced immunity to killing by colicin E1. Of six insertions isolated after conjugation, five fail to produce colicin, are conjugally proficient (transmissible), and map within a 500 nucleotide region of the genome. The other is conjugally deficient, produces colicin normally and maps close to two others with a similar phenotype isolated after transformation. Of four others isolated after transformation, two have similar properties to the original five transmissible plasmids. The other two are nontransmissible and produce colicin. Non-transmissibility is correlated with reduced relaxation complex. Patterns of protein synthesis in minicels by ColE1 and ColE1:: Tn 1 plasmids have been examined: all ColE1 plasmids containing Tn 1 show an altered pattern of ColE1 protein synthesis in addition to three presumptive Tn 1-specified proteins, one of which is shown to be β-lactamase. ColE1:: Tn 1 plasmids can be inserted into the conjugative plasmid R64 drd11 to form a cointegrate in which ColE1 and Tn 1 function can be expressed.