A study among heterosexual men and women with multiple sexual partners was carried out to assess the seroprevalence of antibody against hepatitis C virus (HCV). The 468 participants were recruited among visitors to the Clinic for Sexually Transmitted Diseases in Amsterdam. Sera were tested by an enzyme-linked immunosorbent assay (ELISA; Ortho), a recombinant-based immunoblot assay (RIBA; Chiron), and the polymerase chain reaction (PCR). A total of 468 persons were tested, and seven (1.5%) were found ELISA positive. Another 25 (5%) were ELISA indeterminate. Six of the seven ELISA-positive cases were RIBA positive. Further serum samples from five HCV ELISA-positive persons were tested by PCR, and four were found to be positive. The HCV ELISA-positive/RIBA-indeterminate reaction was PCR negative. None of the 17 RIBA-tested sera of the ELISA-indeterminate group yielded a positive result. There was a good correlation between an ELISA optical density/cut-off ratio greater than 2 and a positive RIBA result. The risk factor for HCV appeared to be the type of sexual partner, i.e., belonging to a "high-risk" group for human immunodeficiency virus infection and origin from hepatitis B-endemic countries. It is concluded that HCV may be transmitted through heterosexual contact but probably with low efficiency.