The autologous mixed lymphocyte reaction (AMLR) is impaired in patients with a number of diseases associated with abnormal immunoregulation. Since allergic disorders are also associated with abnormal immunoregulation, we assessed the AMLR in individuals with a history of ragweed hayfever. Lymphocytes from allergic subjects had a lower AMLR than nonallergic controls by rank order analysis. These results support the association of abnormal in vivo immunoregulation with reduced AMLR.