There is growing empirical evidence that external interaction is an important source of knowledge for business innovation. This paper contributes to the innovation literature by using new measures of interaction to explore the relative importance of external interaction for innovation in Irish high-technology businesses. Based on survey data, the paper finds that external interaction increases the probability of product and process innovation, but the effect is inconsistent across all external interaction agents. Interaction along the supply chain has a positive effect on innovation, and interaction with competitors has an insignificant effect on innovation output. Notably, the paper finds that interaction with higher education institutions has a negative effect on the probability of product and process innovation.