Synchronous cells of the thermosensitive division-defective Escherichia coli strain MACI (divA) divided at the restrictive temperature (42 degrees C) if they were allowed to grow at 42 degrees C for a certain period before protein synthesis was inhibited by adding chloramphenicol (CAP) or rifampicin. The completion of chromosome replication was not required for such divA-independent division. Synchronous cells of strain MACI divided in the presence of an inhibitor of DNA synthesis, nalidixic acid, if they were shifted to 42 degrees C and CAP or rifampicin was added after some time; cells of the parent strain MC6 (div A+) treated in the same way did not divide. These data suggest that coupling of cell division to DNA synthesis depends on the divA function. The ability to divide at 42 degrees C, whether or not chromosome termination was allowed, was directly proportional to the mean cell volume of cultures at the time of CAP addition, suggesting that cells have to be a certain size to divide under these conditions. The period of growth required for CAP-induced division had to be at the restrictive temperature; when cells were grown at 30 degrees C, in the presence of nalidixic acid to prevent normal division, they did not divide on subsequent transfer to 42 degrees C followed, after a period, by protein synthesis inhibition. A model is proposed in which the role of divA as a septation initiator gene is to differentiate surface growth sites by converting a primary unregulated structure, with the capacity to make both peripheral wall and septum, to a secondary structure committed to septum formation.