This paper examines the effects of a professional development programme on the attitudes towards the teaching and learning of teachers in the Anglophone part of Cameroon. The development programme combines a multiplier system with school-based in-service training. The research compares the effects that the training had on the attitudes of three groups of teachers in a control group design: (1) teachers participating in the in-service training, subdivided into (1a) teachers trained as multipliers and (1b) teachers trained by the multipliers, and (2) teachers who did not participate in the training. The study featured a quantitative control group design (n = 292) and is intended to contribute to the discussion on the sustainable effects of in-service teacher training in low-income countries. The data collection was conducted in 13 secondary schools in Cameroon. The results suggest that the programme had effects on the teachers' attitudes and classroom practices.