Rheumatoid factor (RF) induces false-positive results in the detection of serum antibodies, especially of the IgM type. About 70% of the patients with chronic hepatitis C have abnormal levels of serum RF. The aim of this study was to determine whether the presence of serum RF could influence the detection of anti-HCV core IgM, using an assay designed not to pick up RFs by the addition of goat antibodies directed against human IgG in the sample diluent. Serum anti-HCV core IgM antibodies and RF were sought in 60 patients with chronic hepatitis C. Serum anti-HCV IgG antibodies and anti-HCV core IgM antibodies were also sought in 101 patients with high levels of RF. Anti-HCV core IgM antibodies were found in 45% and serum RF in 72% of the patients with chronic hepatitis C. Neither the prevalence nor the levels of RF differed significantly between IgM positive and negative patients. Eight percent of the 101 patients with raised RF had anti-HCV antibodies and two of them had anti-HCV core IgM antibodies. No patient without anti-HCV antibodies had anti-HCV core IgM antibodies. These results show that: a) the detection of anti-HCV core IgM in patients with chronic hepatitis C is independent of the presence of serum RF; b) high titers of serum RF are not responsible for false-positive results of anti-HCV IgM tests. The study suggests that the test used could be a confident tool for studies on the significance of anti-HCV core IgM antibodies in chronic hepatitis C.