A system for examining discontinuous DNA synthesis in vitro on cellophane discs has been developed. Using this system, we will attempt to elucidate the mechanism of initiation of discontinuously synthesized intermediates as well as other biochemical details of discontinuous DNA replication. This work has recently been greatly facilitated by our isolation of a mutant of Escherichia coli temperature sensitive in both the polymerase and exonuclease activities of DNA polymerase I.