The study investigates the validity of ‘subjecthood diagnostics’ in Korean proposed in the previous studies. Based on the studies on subjecthood diagnostics in Korean Single Subject Constructions (non-MSCs) and Multiple Subject Constructions (MSCs), the role of two subjecthood diagnostics―Honorific Agreement (HA) and Plural Copying (PC)―in these two constructions are examined through an empirical syntactic experiment. Seventy Korean native speakers participated in the experiment using acceptability judgment task designed to probe their intuition. The results show (i) that in the Non-MSCs, Honorific Agreement (HA) reliably seems to choose the subject (i.e., the only Nom-marked NP), whereas Plural Copying (PC) does not seem to pick out the subject in a less consistent manner; and (ii) that in the MSCs, neither Honorific Agreement (HA) nor Plural Copying (PC) seems to be a reliable subjecthood diagnostic. Possible reasons for these results are discussed.