From the sera of patients with advanced cancer, a novel factor called SDF (serum-derived factor) was partially purified. SDF was shown to stimulate transcription from the long terminal repeat (LTR) of human immunodeficiency virus type 1 (HIV-1) by transient CAT assay. It did not stimulate gene expression of various control promoters including Rous sarcoma virus, human c-fos, c-myc, c-H-ras and chicken beta-actin genes. The SDF preparation did not contain any detectable TNF-alpha or TNF-beta, and differed in its physicochemical properties from TNFs. We concluded that SDF might be a novel factor associated with the clinical features of advanced cancer. It is speculated that SDF might have some role in disease progression of AIDS as well as in the development of the cachectic conditions in AIDS associated with malignancies.