The plasma beta 2-microglobulin (beta 2-MG) levels in 118 children with thalassaemia were investigated. The mean level was higher than in healthy children. A significant increase of beta 2-MG was associated with hypersplenism (3.14 +/- 0.6 mg/l). The beta 2-MG levels appeared to reflect reticuloendothelial system activity but were not related to iron overload. Fibronectin levels were generally lower than in healthy adults; profound chronic fibronectin depletion was not accompanied by an increased liability to infection.