This study uses audio-recordings of naturally occurring classroom discussions to examine how adult numeracy students drew on their informal knowledge as they worked together to solve mathematical problems. Analysis of the recordings not only reveals the knowledge which the participating students contributed to their discussions, but also illuminates how they responded to, and appeared to value, each other's contributions. Although students were found frequently to share knowledge about curricular and examination requirements, they rarely shared knowledge about out-of-classroom numeracy practices, even when their learning activities gave them opportunities to do so. Drawing on Bourdieu's notions of habitus and field, it is suggested that students' reluctance to draw on their out-of-classroom practices is due to the historically constituted values which they themselves place on different types of knowledge. A typology of "conformative" and "disruptive" knowledge contributions is proposed, which attempts to reconcile transformative ideals with the constraints of the contemporary classroom. (Contains 1 table and 4 notes.)