Agreement is a covariation in morphological form that reflects relations between words. A series of experiments were carried out in Italian during production and comprehension where an element interferes with agreement. The likelihood of interference found in both modalities is related to the markedness of the intervener and to its grammatical nature: it occurs more frequently when the number expressed on a prepositional modifiers intervene between the subject and the verb than the number expressed on an object pronoun. Furthermore, subject agreement process with an intervener object pronoun is prone to error, with many errors reported also in the matched condition and with slower reaction timing in comprehension. The study supports the idea that agreement is a grammatical process sensitive not only to the markedness of the intervener element but also to its structural position. A unifying explanation for agreement in both production and comprehension will be adopted in line with retrieval of an agreement source from a content-addressable memory sensitive to structural positions and their implementation in different languages.