In this paper, I use interviews with two teachers, drawn from a small scale study of four schools, to illustrate the way in which mathematics assessment is used as a form of capital in teachers' wider professional lives and the effect that this is likely to have on teaching practices in mathematics. Assessment is high-stakes for English schools and outcomes are increasingly measured numerically in terms of annual 'pupil progress'. I use an analysis based on Bourdieu to illustrate how this affords competition between teachers for grades and potentially brings them into conflict with each other. I argue that the effect of this kind of professional capital is likely to be unhelpful in relation to what is known to be effective use of assessment in mathematics teaching.