In 1888 Cardinal Lavigerie, the Archbishop of Algiers and Carthage, launched his ‘anti-slavery crusade’. Drawing attention to slave raids in Africa and to the East African slave trade, this initiative resulted in the foundation of several new anti-slavery associations. Many of them maintained close connections to Catholic politics and missionaries, yet also co-operated with two older – and predominantly Protestant – groups in Britain: the British and Foreign Anti-Slavery Society (BFAS) and the Aborigines’ Protection Society (APS). By stressing how anti-slavery associations collaborated through international congresses and periodicals, the article traces the emergence of a new anti-slavery internationalism in the 1880s and 1890s. It discusses the transnational context of such activism and highlights the shared language developed by its protagonists. However, the timing of these efforts makes its necessary to consider the relationship between humanitarianism and the ‘new imperialism’ of this period – a connection that was reflected in diplomatic events such as the Brussels Conference of 1889–90. Anti-slavery discourse fed into notions of a ‘civilising mission’ which seemed to legitimise imperial expansion. Portrayals that contrasted the actions of Western powers with those of so-called ‘Arab’ slave-traders (a term which Europeans also applied to African Muslims) were one element of this discourse.