Teaching experience has been identified as an important factor in pedagogical content knowledge (PCK) development. However, little is known about "how" experienced teachers may draw on their previous experience to facilitate their PCK development. This study examined how two experienced high school biology teachers approached the teaching of a newly introduced topic in the curriculum, polymerase chain reaction and their PCK development from the pre-lesson planning phase through the interactive phase to the post-lesson reflection phase. Multiple data sources included classroom observations, field notes, semi-structured interviews and classroom artefacts. It was found that the teachers' previous experience informed their planning for teaching the new topic, but in qualitatively different ways. This, in turn, had a bearing on their new PCK development. Subject matter knowledge (SMK) can not only facilitate but may also hinder this development. Our findings identify two types of experienced teachers: those who can capitalise on their previous teaching experiences and SMK to develop new PCK and those who do not. The critical difference is whether in the lesson planning stage, the teacher shows the disposition to draw on a generalised mental framework that enables the teacher to capitalise on his existing SMK to develop new PCK. Helping teachers to acquire this disposition should be a focus for teacher training in light of continuous curriculum changes.