This article picks up on a term (‘cross-fertilisation’) often exploited in debates on the interaction of international laws and legal practices, especially in the context of international criminal law. Two questions are addressed: (1) What is the meaning of ‘cross-fertilisation’? (2) What are its conditions? As the article argues, ‘cross-fertilisation’ pertains to the understanding of legal utterances relative to other such utterances. The concept assumes that if an agent wishes to understand the meaning or significance of a legal utterance, his understanding may profit by bringing the analysis of this utterance to bear on its assumed relationship with other legal utterances. Any assumption of a relationship between two legal utterances requires justification, however, or else it will not meet acceptance in international legal discourse. Consequently, when an agent brings the analysis of a legal utterance to bear on its relationship with some other legal utterance, as this article argues, cross-fertilisation will occur on two conditions. First, there has to be recognition of the relationship between the two utterances by a rule, principle, or informal convention pertinent to international legal discourse. Second, the agent must have grasped the precise nature of this same relationship. Based on this proposition, the article ends with six examples illustrating the kind of problems that might obstruct cross-fertilisation proper.