This article addresses the issue of helping students who work collaboratively on mathematical problems with the aim of raising the level of their mathematical understanding and competence. We investigated two kinds of teacher interventions aimed at helping students. The rationale of these interventions was based on a process model for interaction and mathematical level raising. One kind of interventions focused on the interaction between the students, the other--on the mathematical content of the tasks. The effects of the two kinds of interventions were investigated using a pre-test--post-test comparison of students' learning outcomes and analyzing the transcripts of students' verbal utterances and worksheets. Our analyses point to interventions focused on students' interactions as more effective in terms of students' learning outcomes. Theoretical and practical implications of the research are discussed.