ELISA Assay Diluent is a 5x concentrated phosphate buffered saline solution containing (pre-screened) fetal bovine serum as a protein carrier and blocker of antibodies' non-specific binding. 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 Av-HRP or SAv-HRP). The diluent contains a preservative compatible with peroxidase mediated enzyme activity.