Immunization with a peptide fraction from ovarian cancer urine resulted in an antiserum that reacts with a 6,000 dalton (6K) peptide. In urine from some patients with advanced gynaecological cancer, the concentration of 6K peptide was high enough to be detected by immunodiffusion. By radioimmunoassay the levels in normal serum could be determined. The concentrations in serum was 5-20 microgram/l and that in urine 5-50 microgram/l. Elevated levels were observed in urine from 5 out of 8 patients with ovarian cancer, 10 out of 14 patients with cervical cancer and 9 out of 14 patients with endometrial cancer. The highest level observed was 12,000 microgram/l. High concentrations of the 6K peptide (100-300 microgram/l) were observed in amniotic fluid from 14-16 weeks of pregnancy. Some tumor extracts also had a high concentration of the 6K peptide. No immunological relationship was observed between the peptide and known tumor-associated antigens or serum proteins. These results suggest that the peptide is a new oncodevelopmental peptide which may have significance as a tumor marker.