Bartolomeo Vivarini's 1475 Madonna della Misericordia triptych was commissioned by the prestigious Congregazione del Clero for their altar at the ancient church of S. Maria Formosa, Venice. This paper reinterprets the triptych as an important document of Venetian social and ecclesiastical history, and considers the significance of the commission by the priestly sodality. The iconography of the altarpiece is discussed in the light of the form, practices and governance of the under-studied Congregation, and it is proposed that the erection of the altarpiece marked the occasion of the Congregation's espousal of the controversial cult of the Immaculate Conception.