Quo primum

Quo primum (from the first) is the incipit of an Apostolic constitution in the form of a papal bull issued by Pope St. Pius V on 14 July 1570. It promulgated the 1570 edition of the Roman Missal, and made its use obligatory throughout the Latin Rite of the Catholic Church, except where there existed a different Mass liturgy of at least two hundred years' standing.

The declared reason for this measure was this: "It is most becoming that there be in the Church... only one rite for the celebration of Mass." However, he made the exception mentioned, which permitted the survival, within limited areas or in celebrations by members of certain religious orders, of Latin liturgical rites other than the Roman Rite, rites such as the Ambrosian and Mozarabic Rites, that of the Diocese of Lyon and certain Catholic Order Rites. Some of these dioceses and religious orders have since decided to adopt the Roman Rite. Others preferred not to avail themselves of the exemption to which they were entitled and instead to adopt the Roman Missal immediately.