At the present time relatively little is known of the steps in the synthesis of DNA. Recent biochemical, physical and genetic evidence indicate that the control of DNA synthesis in T-even phage-infected cells and cancer cells is considerably more complex than had been assumed. The proposed project is designed to continue studies whose aims are: (1) to gain a general understanding of the biochemical and genetic mechanisms controlling DNA synthesis; (2) to obtain information on the control of deoxyribonucleotide biosynthesis of bacteriophage T4 and of mammalian KB cells; (3) to establish the functions of genes that are essential for the continuation of DNA replication after it is initiated.