Bacteriophage T4 deoxyribonucleic acid (DNA)-protein complexes were retained preferentially on glass fiber filters. DNA polymerase activity in the complex was detected through the incorporation of (3)H-labeled DNA precursors. The primer-product DNA hybridized with both phage and Escherichia coli DNA. Density labeling experiments showed that about 30% of incorporated (3)H-deoxyadenosine triphosphate was found in DNA which hybridized with phage DNA; this DNA was found to be covalently attached to the primer DNA.