Plasma Sample Diluent is formulated specifically for use with plasma samples being tested in an antigen-down ELISA format. This novel mammalian-protein-buffered formulation provides an assay environment that minimizes non-specific binding while maintaining proper ionic strength and pH for efficient antibody-antigen interaction. Plasma Sample Diluent also inhibits thrombin activity (clotting) and complement activity during the plasma sample incubation period.