A side-by-side comparison of the different neutralization assays used for characterization of these patients' plasma and the newly isolated Abs showed that the broadest spectrum of strains and subtypes was neutralized in the PV assays as well as in the 24/1/ 14 extended incubation PBMC assay with primary virus. In contrast, the classical short incubation phase assays as well as the extended absorption phase PBMC assays showed a reduction in the number of neutralized strains. The TZMbl assay using primary virus also showed this restricted profile despite the fact that it has an extended absorption phase in common with the cell line PV assays. Results for the three patient plasma that were selected for their cross-neutralizing capacity in the 24/1/14 PBMC assay correlated with those obtained in the TZMbl_PV assay for only two patients. It is unusual that these two substantially different techniques result in comparable neutralization profiles (own results and [20] ). Both PV based assays correlated strongly with each other.