The branched DNA (bDNA) assay was compared with a semi-quantitative cDNA-polymerase chain reaction (cDNA-PCR) assay for monitoring HCV RNA levels in plasma in 17 haemophilia patients participating in a controlled alpha-interferon trial. Good correlation between the HCV RNA levels as detected by the two assays was observed, with a correlation co-efficient of 0.83 (P < 0.0001) and 0.90 (P < 0.0001) at week 0 and 24, respectively. Hepatitis C virus RNA (HCV RNA) levels could be assessed with the bDNA assay in 14/17 (82 percent) HCV cDNA-PCR positive pretreatment samples. The bDNA assay apparently failed to detect low viral titres. Interferon treated patients (n = 11) showed either a complete response, being a large reduction in HCV RNA level to below the detection limit of the HCV cDNA-PCR assay (6/11) or no significant reduction in HCV RNA level (5/11). A "partial" virological response was not observed. The changes in HCV RNA plasma levels in non-responders during interferon (IFN) treatment were similar to the (small) natural fluctuations in viral load observed in controls (untreated patients). Although the bDNA assay was not as sensitive as cDNA-PCR, given its user friendliness and quantitative results, it is concluded that it is a useful test for monitoring HCV RNA levels in patients treated with interferon. However, patients who are non-reactive in the bDNA assay have to be retested by cDNA-PCR because low viral titres are not detected by the bDNA assay.