Highly selective affinity labeling of a DNA-polymerase α-primase complex from human placenta by o-formylphenyl esters of ATP, ADP and AMP was performed in a two-step procedure in which a substrate analog attached to the active center was elongated by radioactive ATP. If the covalent attachment is performed in the presence of poly(dT) template, the ATP esters modify selectively the δ subunit of the complex. If poly(dT) is added after the covalent binding of the reagent, both δ and γ subunits become labeled. With the o-formylphenyl ester of AMP the δ-subunit is modified. The ADP ester modifies both the δ and γ subunit in the presence and absence of template. It is shown that formylphenyl ester of ATP is not the substrate in the reaction of elongation catalyzed by primase. The data obtained suggest the binding site of initiating substrate to be located in the region of contact of the two subunits of primase. The role of the template in the formation of the active site is discussed.