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Highly sensitive reporter-gene assays have been developed that allow the precise quantification of both the direct vascular endothelial growth factor-A neutralizing activity of bevacizumab and the ability of bevacizumab to activate antibody dependent cellular cytotoxicity. The use of these assays to analyzes samples from patients with ovarian cancer following four cycle of bevacizumab treatment revealed a close correlation between bevacizumab neutralizing activity and antibody dependent cellular cytotoxicity activity, and a reasonably good correlation between both activities and circulating drug levels determined using an enzyme-linked immunosorbent assay. Analysis of longitudinal samples from a small cohort of patients with glioblastoma treated with bevacizumab revealed a lower correlation between these parameters. We report herein that reanalysis of the grouped samples from the two studies using the nonparametric Spearman rank correlation coefficient revealed a surprisingly good correlation between the two facets of bevacizumab activity, and between both activities and circulating drug levels despite the different indications and treatment regimens, revealing new insights into the action of bevacizumab in neoplastic disease.