AbstractAn enzyme‐linked immunosorbent assay (ELISA) was developed based on sera from patients convalescent from non‐A, non‐B hepatitis and haemophilia A patients who had been frequently treated with commercial blood products. Using this ELISA, an antigen was detected which appears to be related to non‐A, non‐B hepatitis. The antigen is provisionally designated as DS‐antigen (DS‐Ag). The serum of another patient with haemophilia A, which was strongly positive for the DS‐Ag, caused a typical case of non‐A, non‐B hepatitis in a chimpanzee. DS‐Ag could be detected in the serum of the chimpanzee during the acute phase of the infection.The ELISA for DS‐Ag reacted with neither hepatitis A or B virus antigens, nor Epstein‐Barr virus or cytomegalovirus. The assay was provisionally evaluated using sera from different groups of patients. Out of 17 patients with posttransfusion hepatitis non‐A, non‐B, 11 were found positive in the ELISA for DS‐Ag (65%).As expected, a relatively high prevalence of DS‐Ag (9%) was found in patients with haemophilia, who are regularly treated with blood‐clotting factor‐concentrates. Antibodies to DS‐Ag were found in 48% of these patients. The DS‐Ag was found in 8 of 1400 (0.6%) volunteer blood donors, and antibody to DS‐Ag in 3% of the sera. Remarkably, a high incidence (41%) of antibodies to DS‐Ag was found in prostitutes, suggesting that this antigen may be transmitted by a sexual route.The DS‐Ag was pelleted by ultracentrifugation for four hours at 100,000g and was found to have a