The formation of the Cominform in 1947 was a decisive moment in the Cold War. Although many rank-and-file activists in the Labor and Communist parties in Great Britain and Australia continued to cooperate with each other, the formal relationship between the two parties sharply deteriorated. In Britain, the formation of the Cominform shattered the Communist Party’s hopes of post-war class peace. Communists’ critical attitude to the Labour Party became openly hostile. However, no fundamental change to Communist Party policy occurred. In industry, the Party became more militant but, generally, continued to pursue an approach that involved collaboration more than confrontation. In Australia, the situation was different. Cominform perspectives significantly altered the position of the Communist Party, which shifted from conciliation to intransigence, from a desire to cooperate with the Labor Party to an intention to ‘liquidate’ reformism. Enmity was mutual: influenced by both the Cold War environment and the increasingly powerful anti-communist Industrial Groups, the hostility of Labor to communism became palpable. The article examines the post-war decline of both communist parties in the context of the interplay between Communist Party policy, Labor Party antagonism, and the international environment of the early Cold War.