The article focused on the use of assessment for learning in promoting active learning and learner participation in mathematics. Assessment for learning (AfL) has been found to enhance learning and improve performance. However, teachers' use of AfL to enhance active learning has not been clearly outlined. This study is part of the broader research study that explored mathematics teachers' use of AfL to enhance mathematics teaching and learning in primary schools in Alexandra Township, Johannesburg. A case study research-type and a qualitative approach were used to collect data from mathematics teachers. Nine teachers were purposefully selected from whom data were collected using semi-structured interviews and non-participant observation. The findings revealed that teachers had limited pedagogical knowledge in using AfL to promote active learning in their classrooms. They failed to apply a learner-centred approach that promotes effective learner participation in mathematics classrooms. Therefore, it is recommended that teachers undergo ongoing continuous development on classroom time management and planning for the effective use of AfL.