The sensitivity of ORTHO™ HCV 3.0 ELISA Test System (ELISA 3) for the detection of anti-HCV was compared with the second-generation ELISA, ORTHO HCV 2.0 ELISA Test System (ELISA 2). ELISA 3 differs from ELISA 2 in that it incorporates the HCV recombinant antigen NS5, in addition to recombinant antigens derived from the NS3, NS4 and core regions of the HCV genome. Specimens tested consisted of serial bleeds obtained from 21 individuals undergoing seroconversion following acquisition of post-transfusion HCV infection. ELISA 3 demonstrated significantly greater sensitivity than ELISA 2, detecting seroconversion earlier in 24% (5/21) of cases. Although one of these cases appeared to represent early seroconversion to NS5, most of the improved sensitivity of ELISA 3 appeared to derive from increased detectability of anti-c33c.