Control serum samples of young, healthy volunteers (N = 5) were compared with serum samples and laryngeal cancer extracts of patients (N = 12) with stage 3 and 4 cancers. Eight patients were primarily treated by total laryngectomy, 4 patient by palliative irradiation therapy (60 Gy). Ethanol (80%, pH 2) extracts of sera and cancer specimen were gel chromatographed, and the nominally 0.3-5 KDa molecular mass substances were further examined by analytical capillary isotachophoresis (ITP) (LKB. Sweden). Serum samples were also examined after surgery of irradiation therapy. Serum and tumor extracts of each laryngeal cancer patient contained a substance in the molecular mass range of 0.3-5 kDa. This was not detected in the serum of normal control subjects. The marker-candidate substance disappeared from the serum of four out of 8 laryngectomized patients who live tumor-free for years. Recurrence or lymph node metastasis developed in three of the remaining four patients who did not become free of the marker substance after laryngectomy. The concentration of the marker has been increased in the serum after palliative irradiation in all four patients. This marker substance is probably a product of tumor necrosis. The marker may adequately monitor the recurrence of tumor postoperatively of after irradiation therapy.