Mauvais Sang (French pronunciation: [movɛ sɑ̃], Bad Blood), also known as The Night Is Young, is Leos Carax's second film. Released in 1986, the film played at the 37th Berlin International Film Festival before being nominated for three César Awards and winning the Prix Louis-Delluc. The film sold 504,803 tickets in France. The title refers to Arthur Rimbaud's poem in A Season in Hell.

In the film, a sexually transmitted disease called STBO is sweeping the country; it spreads by sex without emotional involvement, and most of its victims are young people who have sex out of curiosity rather than commitment. A woman hires two men to steal the serum, which has been locked away in a pharmaceutical company's offices.