Abstract
Nalidixic acid has been used to interrupt DNA synthesis in exponentially growing cultures of E. coli B/r. Within three minutes after removal of the nalidixic acid block, premature rounds of chromosome replication are initiated at the normal origin of vegetative DNA synthesis. The new growing forks traverse the chromosome in about 40 minutes, the normal replication time. These rounds continue synchronously into the second generation after removal of the nalidixic acid. These phenomena are similar in some respect to the premature initiation of chromosomal replication observed after thymine starvation or after release of thermo-sensitive DNA synthesis mutants from the restrictive temperature that prevents DNA synthesis.