45, is a German-language Zeitoper with words and music by Austrian composer Ernst Krenek about a jazz violinist.

The work was quickly adopted by opera houses across Germany, where it was performed 421 times on various stages during its first season alone.

The arietta from act 1, "Leb' wohl, mein Schatz", was as an arrangement for jazz band or salon orchestra released as "Jonny’s Blues" on several 78 rpm recordings.

The magazine Vogue described the scene in spring of 1928: In Munich, for instance, the opera-house was closed to it, and, to be presented at all, it had to go to the Gaertner Platz Theater.

Jonny slips into the room, sees Daniello's Amati violin and decides to steal it but is prevented from doing so by Anita's arrival.

Jonny observes this and, with the help of key he has obtained from Yvonne, steals the violin and hides it in Anita's banjo case.

The next morning Anita leaves Daniello's room to meet with her manager and sign a contract that will take her over to America to sing there.

Daniello then discovers his violin is missing, realizes someone has to have had access to his room and summons the hotel manager who sacks Yvonne.

Shortly after, Yvonne arrives, tells Max about Anita's love life in Paris and shows him the ring.

The radio station plays another track, this time of Jonny's jazz band and Daniello, who is also listening to the broadcast while staying in another hotel recognizes the sound of his Amati violin.

In the ensuing confusion Yvonne accidentally pushes Daniello off the platform and he is killed by an incoming train.