The spatial relationship between the polymerase and exonuclease active sites of bacteriophage T4 DNA polymerase enzyme has been examined using a bulky biotin-streptavidin block at a specified position in an oligonucleotide (Fig. 1). The idea was to monitor the closest distance of approach of the T4 enzyme before being blocked by the bulky biotin-streptavidin complex while performing either of its activities. The results indicated a distance of 4-5 nucleotides between the biotin-streptavidin probe and the exonuclease site and a distance requirement of at least 7 nucleotides between the bulky probe and the 3'-primer terminus for efficient polymerization by the T4 enzyme. The difference in the two distances suggested a separation of 2-3 nucleotides between the two active sites of the T4 enzyme.