ELISA Assay Diluent is a 5x concentrated phosphate buffered saline solution containing (pre-screened) bovine serum. This buffer is specifically formulated and tested for ELISA applications, such as blocking antibody-coated ELISA plate microwells, diluting ELISA standards and samples, and diluting biotinylated detection antibodies or tertiary reagents (specifically Avidin-HRP or Streptavidin-HRP). The diluent contains a preservative compatible with peroxidase mediated enzyme activity.