1. The reaction between crystalline horse serum albumin and homologous antibody in rabbit sera is quantitatively accounted for by expressions similar to those derived from the law of mass action for other immune precipitating systems. 2. The reaction of an azo dye prepared from crystalline serum albumin by coupling with diazotized R-salt-azo benzidine was also studied with homologous antibody and anti-serum albumin. 3. Quantitative data obtained on cross reactions with the two antigens differ markedly from data on the corresponding reactions in the egg albumin system and indicate that tyrosine and perhaps histidine, while important in determining the serological specificity of egg albumin, have little connection with the specificity of serum albumin. 4. Calculations are made of the equivalent composition of the specific precipitate at various reference points in the reaction range.