Cellular immunity plays a key role in the defence against the larva of the cestode Echinococcus multilocularis. This larva is responsible for alveolar echinococcosis (AE) of the liver, a rare parasitic disease which occurs in endemic areas including European alpine countries, Alaska, the USSR, Western China and Northern Japan. We have shown a marked decrease of the CD8+ T-cell population in the blood and we have described an infiltrate composed mainly of activated CD8+ T-cells in the liver lesions of most patients with AE. In this study, we assessed the serum level of soluble IL-2-receptor (sIL-2R) and CD8 (sCD8) in 37 patients (23 men, 14 women, mean age 59.5 yrs) with a histologically proven AE. The results, obtained using sandwich ELISA, were compared to those of healthy controls and correlated to parameters evaluating the severity of the disease. The mean serum levels of sIL-2R were significantly higher in AE patients than in controls. There was a significant correlation between sIL-2R levels and both the volume of parasitic lesions and a calculated index of severity of the disease. The mean serum levels of sCD8 did not differ significantly from the values obtained in controls. These results indicate that the infiltration of the liver by CD8+ T-lymphocytes is not associated with an increased release of sCD8 into the serum. The circulating levels of sIL-2R appear to reflect the extent as well as the severity of the disease. Immunostaining of the cells of the periparasitic granuloma suggests that the cell origin of the sIL-2R could be macrophages rather than T-lymphocytes.