Alfalfa sprouts can induce systemic lupus erythematosus (SLE) in monkeys. This property of alfalfa sprouts has been attributed to their non-protein amino acid constituent, L-canavanine. Occurrence of autoimmune hemolytic anemia and exacerbation of SLE have been linked to ingestion of alfalfa tablets containing L-canavanine. In this report we show that L-canavanine has dose-related effects in vitro on human immunoregulatory cells, which could explain its lupus-inducing potential. These effects include: 1 diminution of the mitogenic response to both phytohemagglutinin and concanavalin A but not to pokeweed mitogen, as determined in both thymidine incorporation and cell cycle studies, and 2 abrogation of concanavalin A-induced suppressor cell function, which results in increased release of both IgG and DNA binding activity into supernatants by cells from normal subjects and SLE patients. These immunoregulatory effects of L-canavanine may explain the induction or exacerbation of SLE by alfalfa.