Sera from 50 patients with Hodgkin's disease, 78 patients with non-Hodgkin non-leukemic malignant lymphomas, and 75 patients with different types of solid malignant tumors were investigated for the presence of immune complexes using the (125I) C1q-binding test. All patients were untreated. An increased serum C1q-binding activity was found in 22% of the patients with Hodgkin's disease, 35.9% of the non-Hodgkin lymphoma patients and in 37.3% of the patients with solid tumors. The C1q-binding material detected in the patients' sera had properties similar to those of immune complexes. On sucrose density gradient it sedimented as a 10-30 s material. It contained IgG which were dissociated under acid conditions. Passage through anti-IgG immunoabsorbent removed its C1q-binding properties. A prevalent association was found between the presence of serum immune complexes and disseminated disease stages in all the patient groups included. A similar association was found between the presence of serum immune complexes and general symptoms among the malignant lymphoma patients. The nature of the antigens involved in the complexes remains unknown.