The ongoing JANUS project was started in 1973. The serum bank comprises 424,938 serum samples consolidated from 293,692 donors. The specimens are stored at -25 degrees C. From 1 to 13 consecutive samples are available from each donor. Up to October 1993 about 14,000 of the donors had developed some form of cancer. Frozen serum samples collected from a few months to 19 years prior to clinical recognition of their disease are available for research purposes. The principle aim of the JANUS project is to search in the premorbid sera for chemical, biochemical, immunological or other changes that might be indicative of cancer development at early stages. Gas chromatography-mass spectrometry and two-dimensional protein electrophoresis have been used to evaluate the stability of the frozen sera. Some recent findings are: CA-125 may be elevated months prior to the diagnosis of ovarian cancer; serum thyroglobulin may be a preclinical tumor marker in subgroups of thyroid cancer; low levels of selenium in serum reflects increased risk of thyroid cancer; raised antibodies in serum against Epstein-Barr virus is a risk factor for development of Hodgkins disease; prostate-specific antigen may be elevated years prior to clinical diagnosis of prostate cancer; and linoleic acid in serum phospholipids is inversely related to breast cancer risk. The serum bank is, in principle, suitable for environmental studies, e.g., human exposure assessment. The steering committee of the JANUS project is open to suggestions for collaborative research on this topic.