A mutant of Bacillus subtilis unable to initiate a new round of replication at 45 C has been described. Here we show that inhibition of DNA synthesis in this mutant is reversible and that DNA synthesis is resumed at low temperature, even in the presence of chloramphenicol. Initiation of a new replication cycle thus can occur in the absence of protein synthesis. A thermolabile component required for initiation therefore appears to be synthesized at 45 C in an inactive form and can be activated at 30 C in the presence of an inhibitor of protein synthesis. Although resistant to chloramphenicol, the reinitiation of replication occurring after lowering the temperature is sensitive to rifampin and streptolydigin.