The domestic policies and excesses of American anti-Communism in the early Cold War, dominated by the image of Senator Joseph McCarthy, have been the subject of controversy and a great deal of historical research. No less significant and much commented on at the time was the influence of McCarthyism on foreign relations and perceptions of the United States abroad. This article deals with the British government's responses to the anti-Communist fervor in the United States in the late 1940s and early 1950s. Britain was itself grappling with the challenges posed by the Cold War, including those relating to security and civil liberties in a democratic society. The impact of American anti-Communism was felt strongly in that context. The article draws extensively on recently released files from the British security services and other British government agencies.