Antigen recognition by the adaptive cellular immune system is based on a diverse repertoire of antigen receptors. Since this repertoire is formed by genetic recombination, a number of receptors are autoreactive by chance, giving rise to the threat of autoimmune disease. Potentially autoreactive T lymphocytes (T cells) are rendered ineffective by various tolerance mechanisms. One of these mechanisms is negative selection, the deletion from the repertoire of immature autoreactive T cells in the thymus. The present paper shows how to assess the contribution made by negative selection relative to other tolerisation mechanisms by deducing the impact of negative selection on the T cell repertoire from the statistics of autoantigen presentation in the thymus.