For the initial characterization of a recombinant plasmid, it is necessary to determine the size of the plasmid or, preferably, the size and characteristics of the insert itself. A method is therefore required for the simultaneous preparation, from a number of isolates, of plasmid DNA in a state sufficiently pure for restriction enzyme digestion. The requirements of such a procedure are: (i) A simple method for rapid lysis of the bacterial cells. (ii) Separation of plasmid from chromosomal DNA. (iii) Removal of proteins and of other components of the cells that might interfere with restriction enzyme treatment. (iv) Removal of detergents, salts, etc. used in the process.