Treatment of human plasma with methylene blue in combination with visible light (MB/light) inactivates several bloodborne viruses such as retro viruses and herpes viruses. The viral nucleic acid is thought to be a critical target for the inactivation procedure. We investigated the effects of photodynamic treatment on the RNA of hepatitis C virus (HCV) and human immunodeficiency virus type 1 (HIV-1) using Amplicor reverse transcriptase polymerase chain reaction (RT-PCR), which detects and quantifies a small fragment of the viral RNA. The detectable HCV RNA load (5-nontranslated region) in infected human plasma declined by 94-97 % within 10 min of illumination in small-scale experiments (1-2 ml vol.). Since the same effect was observed in both anti-HCV positive and negative ("window") samples, it can be concluded that HCV antibodies do not influence virus inactivation by photodynamic treatment. The effect of treatment on RT-PCR signals of HIV-1, which is known to be inactivated rapidly by MB/light treatment, was examined. Plasma was infected with HIV-1 and subjected to RT-PCR, which detected a part of the gag gene. The extent and kinetics of PCR signal reduction induced by MB/light treatment were similar to those observed for HCV. Experiments at production scale where single plasma units (300 ml) were infected with HCV showed reduction rates of PCR signals consistent with those measured in the small-scale experiments. The data support the view that MB/light treatment affects the viral nucleic acids and suggest that HCV is susceptible to the procedure.