Milko Kelemen (b. 1924) is one of the most extraordinary Croatian composers of the post-World War II era. He has received numerous prestigious awards for his work, while his compositions are published by major companies, including Schott, Universal, Peters, and Hans Sikorski editions. His music is little studied or known internationally. This paper examines this innovative, avant-garde musician and sheds light both on Kelemens life and his compositional technique. The latter is examined in four compositions featuring cello as the main subject: Changeant (1968), Drammatico (1983), Requiem for Sarajevo (1994), and Musica Amorosa (2004). The examination of these compositions is placed in a biographical context which reveals how Kelemens keen political and cultural interests influenced his development as a composer. In particular, this document looks at Kelemens role as one of the founders, and the first president, of the Zagreb Music Biennale Festival, an event known for its ground-breaking work in bringing together artists and composers from the eastern and western blocs during the Cold War, as well as revitalising Croatias old-fashioned and provincial cultural scene. This festival of avant-garde music has been running since 1961.