Previous findings indicate that the effectiveness of in-service teacher training can be increased by a (pedagogical) content knowledge orientation on the one hand and the stimulation of cooperation among teachers on the other. In this paper, three versions of a multi-phase training program were compared in a quasi-experimental field study with a pre-post follow-up design: (A) a pedagogical content-focused version; (B) a version that especially stimulated cooperation; and (C) a version that combined both aspects. Additionally, two control groups D and E were investigated. Results show that trainings A and C were evaluated more positively, or at least not worse, than training B and the control groups D and E with regard to (1) the "acceptance of the training" in terms of the promotion of process-related competences as well as of cooperation, (2) the "acceptance of the innovation," and (3) teachers' "epistemological beliefs."