The E. coli minicell locus (minB) was shown to code for three gene products (MinC, MinD, and MinE) whose coordinate action is required for proper placement of the division spetum. Studies of the phenotypic effects of expression of the three genes, alone and in all possible combinations, indicated the following: cell poles contain potential division sites that will support additional septation events unless specifically inactivated; the minC and minD gene products act in concert to form a nonspecific inhibitor of septation that is capable of blocking cell division at all potential division sites; and the minE gene codes for a topological specificity factor that, in wild-type cells, prevents the division inhibitor from acting at internal division sites while permitting it to block septation at polar sites.