The article analyzes professional collaboration in a lesson study involving university mathematics professors, with the objective of contributing to professional development. The research was guided by the question, What principles promoted professional collaboration among university mathematics professors participating in a lesson study? The research focused on a lesson study involving university professors of mathematics and mathematics education over twelve weekly meetings of two hours each. The empirical material included the researchers' field notes and the transcripts of the recordings of the lesson study sessions. A qualitative analysis, based on content analysis, revealed that collaboration involved: "professional sharing" and "decision-making." The research shows that by fostering collaboration the lesson study mobilized different principles of teaching professionalism, such as knowledge that is basic to teaching, values and ways of conducting university teaching, a social and moral commitment to teaching, and teaching autonomy, contributing to the professional development of higher education professors.