The paper defines and analyzes the morphosyntactic properties of first-conjunct agreement, which arises when an adjective or verb agrees with the highest/first conjunct of a coordinate noun phrase. This agreement pattern is derived by means of the syntactic operation Agree and a new postsyntactic mechanism which acts as a filter on Vocabulary Insertion within the framework of Distributed Morphology. The proposed filter is called Vocabulary Item Feature Harmony, and roughly consists of (phi-)feature identity between Vocabulary Items. The biaspectual analysis, and especially feature harmony, is used to understand and account for gradable and variable acceptability of first-conjunct agreement, as well as the distribution of this agreement pattern in relation to another agreement pattern, namely, masculine plural agreement (with the coordinate phrase as a whole). The investigation is focused on Bosnian/Croatian/Serbian first-conjunct agreement, but the findings could be extrapolated to similar cases in other languages.