A series of 120 patients with chronic delta hepatitis virus (HDV) were investigated using a newly developed assay for the detection of serum delta RNA and this marker was correlated with other markers of HDV infection. The assay was shown to be both specific and sensitive and provides a direct non-invasive measurement of HDV infectivity. Serum HDV RNA was detected in 51.2% of all patients and in about 64% of those who were liver HDV antigen positive. Its presence was particularly associated with the early stages of the disease where it was found in 83% of cases with chronic active hepatitis (CAH) and progressively less common in CAH associated with cirrhosis and in inactive cirrhosis. The presence of both HBeAg (and HBV DNA) and high levels of HDV RNA in the sera of 5 of the patients analysed, clearly demonstrates simultaneous replication of both HBV and HDV. The serum HDV RNA 'slot blot' assay described in this study should prove invaluable in elucidating further the natural history of delta hepatitis and in monitoring antiviral therapy.