Musical Mutual Protective Union.

Although some musicians' organizations began to appear in the latter half of the 19th century, the MMPU, established in 1863, was the first organization that could be classified as a trade union.

It affiliated with the National League of Musicians which was established in New York City in 1886. It is unclear whether the MMPU had previously affiliated with the NLM's short-lived predecessor, the National Musical Association (ca. 1871-79). In November 1896, after the NLM repeatedly refused to affiliate with the AFL, Samuel Gompers organized the American Federation of Musicians. By 1904, the NLM was drived out of existence. The MMPU played a key role in the NLM and was the leader in the fight against AFL affiliation. The AFM chartered a local in New York City comprised of a band of small locals outside of the MMPU. This local, Local 7, was soon expelled from the AFM and a new local, Local 41, was chartered and began to recruit MMPU members. Local 41 came into existence with 400 MMPU members as opposed to the MMPU's 3,000 members. By 1901, however, 1,000 MMPU members had joined Local 41. Local 310, made up largely of independent-minded MMPU musicians, found itself in opposition to the AFM on many occasions but because a large number of the country's most visible musicians and musical establishments were influenced by 310, the AFM could do little to challenge its leadership.

Local 310 was finally suspended by the AFM in July 1921 for refusing to receive transfer cards from other locals. It was replaced by Local 802 in August 1921.