The HCV (hepatitis C virus) Serotyping 1-6 As-say™ (Murex Laboratories) was evaluated on 303 French HCV-infected patients. Serological typing results were compared to the genotypes obtained from sequence analyses of the 5Ј noncoding regions of the virus genome from 46 HCV-infected patients, and assay specificity was found to be high (97.6%). The serological typing assay, run in 257 consecutive HCVinfected patients, yielded an assay sensitivity lower (70.6%) than that previously reported. This finding was attributed mainly to nonreactive sera from human immunodeficiency virus (HIV)-positive patients (P < 0.001) and perhaps reflected cryoglobulin positivity in others. No anti-type 6 reactivity was detected, and the overall serological type distribution values for types 1 to 5 were 67.3, 7.9, 16.4, 6.6, and 0.9%, respectively. A higher prevalence of type 4 was noted among HIV-infected patients (P < 0.001). In addition, serotype 2 was significantly more frequent in cryoglobulinemia positive than in cryoglobulinemia-negative patients (P < 0.05). Although an initial high level (7%) of mixed serological typing reactivities was found, after predilution of serum only two mixed infections could be confirmed (0.9%). It is suggested, therefore, that mixed reactivities have to be interpreted carefully and retested with prediluted serum, particularly when the optical density of the reactivity is >2.5 or remains >0.4 after competition with all type-specific peptides. The high specificity and relatively good sensitivity even in immunocompromised patients obtained with this assay indicate that it can be used routinely. Because response to treatment is linked to HCV type, this assay could be used to identify HCV serotype to guide therapeutic decisions.