In the presence of single-stranded DNA, the bacteriophage T4 gene 41 and gene 61 proteins catalyze the synthesis of a group of pentaribonucleotides which are homogeneous in chain length but heterogeneous in nucleotide sequence. When single-stranded T4 DNA is used as template, a unique dinucleoside sequence, pppApC, is found at the 5' end of these pentaribonucleotides with the general sequence pppApCpNpNpN. In the presence of the remaining five T4 replication proteins, the pentaribonucleotides can be utilized with high efficiency to prime de novo DNA chain starts; as a result, the vast majority of them can be detected at the 5' end of newly made DNA molecules in an unaltered form. There are multiple, but specific, sites at which new DNA chains are primed in this way on a natural single-stranded DNA. Because identical RNA primers have been isolated from the 5' end of the Okazaki fragments made in T4-infected cells, we suggest that the T4 gene 41 and gene 61 proteins also make the pentaribonucleotides that prime de novo T4 DNA chain starts in vivo during lagging strand DNA synthesis.