Ferritin concentration has been measured in the serum of patients with Hodgkin's disease (HD) by radioimmunoassays with monospecific antibodies to liver (basic) and HeLa (acidic) ferritin. Elevated levels of serum ferritin with the liver ferritin assay were found only in patients with systemic disease, and were associated with low serum iron. Basic ferritin levels returned promptly to normal when complete remission was achieved. High levels of serum ferritin with the HeLa ferritin assay were found in 94% of all untreated patients. Acidic ferritin concentration was not related to systemic symptoms or alterations of iron metabolism, and returned to within the normal range only 1–2 yr after complete remission. These findings suggest that basic and acidic isoferritins can be distinguished in terms of biological and clinical significance. Basic ferritin is synthesized by the reticuloendothelial cells and the high values found in patients with systemic symptoms are compatible with the non-specific changes known to occur in the reticuloendothelial system during inflammation. In patients with untreated HD an elevated serum concentration of basic ferritin can be considered a marker of systemic symptoms and, therefore, an unfavourable prognostic factor. Acidic ferritin may be derived from abnormal lymphocytes and/or monocytes, including malignant cells, and its serum concentration may be of value in following the course of remission.