SummaryOptimal concentration of sodium chloride for quantitative evaluation of staphylococcal agglutinins was 1.3% to 1.7%. This applies to rabbit and human antisera. At this concentration, macroscopic technics are reliable and spontaneous agglutination is inhibited. The quality of the reaction is best at these concentrations for human antisera, while in rabbit antisera, it is best at concentrations of 3.2% sodium chloride. Concentrations below the optimal dissolves the antigen to such extent that macroscopic agglutination is definitely decreased.