Measurement of the soluble form of CD8 antigen (sCD8), a surface membrane component of suppressor/cytotoxic T cells, has yielded useful information relevant to prognosis in children with acute lymphoblastic leukaemia and Hodgkin's disease (HD). An ELISA technique was used to measure the amount of sCD8 in sera from 123 adults with untreated HD. Significantly higher mean sCD8 levels were found in patients with advanced disease (stage III-IV; P less than 0.001), B-symptoms (P less than 0.001), male sex (P less than 0.05) and increased spontaneous and decreased Concanavalin A induced blood lymphocyte DNA-synthesis (P less than 0.05). Actuarial survival of patients with high sCD8 levels was significantly worse than that of the remainder (P less than 0.05). However, the sCD8 level did not add prognostic information to that achieved by age, sex, lactate dehydrogenase (LDH) or clinical stage. A significant correlation between the sCD8 and LDH levels (r = 0.33; P less than 0.001) and inverse correlations between sCD8 levels and total blood CD4+ (r = -0.52; P less than 0.001) and CD3+ (r = -0.39; P less than 0.01) cell counts were found. Ten patients were also studied in complete remission, showing a significantly reduced sCD8 level in comparison to the pretreatment value (P less than 0.05). Increased sCD8 in HD may indicate enhanced suppressor T cell activity possibly compromising the host disease balance which could explain the association with prognosis.