Two new mutants dbf3 and 4, are specifically defective in DNA synthesis. When synchronous cultures of dbf4 were transferred from the permissive to restrictive temperature before the start of the S phase, no DNA synthesis occurred. However when switched after the beginning of DNa replication, the cells completed that round of synthesis. Dbf4 therefore resembles cdc7, a mutant believed to be required for initiation of DNA synthesis, and indeed dbf4 acts at a point in the cell cycle close to cdd7, namely between cdd4 and the final requirement for protein synthesis before the S phase. Like dbf4, cultures of dbf3 transferred to restrictive conditions before the start of S showed little DNA synthesis. However a small burst did occur at a time roughly corresponding to when normal initiation would be expected. Cultures switched after this time completed the ongoing round of DNA synthesis.