The first composition Saint-Saëns completed after marrying Marie Laurie Emilie Truffot on February 3, 1875, was the Allegro appassionato in B minor for cello and piano or orchestra, Op. 43. It was published soon after in 1875. Because of its lighthearted brevity and its appearance at roughly the same time as much larger works such as the Piano Concerto No. 4, Op. 44; the Danse macabre, Op. 40; and the Piano Quartet in B flat major, Op. 40, the Allegro appassionato almost seems like an afterthought. However, it remains one of the composer's most popular works, and the piece's showcasing of the cello makes it a favorite among performers.

Transparently scored and elegantly tuneful, the Allegro appassionato's lively rhythms create a mood similar to that of the Cello Concerto in A minor, Op. 33. Undoubtedly, the piece's melodiousness and numerous repeats of minimal material are the reasons for its popular success.

Immediately after a few syncopated chords in the orchestra, the solo cello enters with a bouncy tune; the entire theme and then the second half are repeated, creating a melodic pattern heard in much of Saint-Saëns' music. The B section of the piece features a much more lyrical melody that opens with a large, aggressive leap in the cello. Section A returns but is abbreviated, giving way to the B material, this time in a new key. An extension of the B section tune leads back to a fiery repetition of section A that contains a reference to the aggressive leap of part B, while a spirited coda closes the piece with rapid triplets. Although only a few minutes in length, the Allegro appassionato makes an indelible impression.

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