1. The effect of human and rabbit sera on the cataphoretic mobility of glass and quartz particles, and of paraffin oil droplets, was studied in serum dilutions (with 0.85 per cent NaCl) from 1:50 to 1:1,000,000, over a pH range of 3.6 to 9.3. 2. Under the conditions described, these various types of particles adsorbed protein partially or completely from the most dilute solution giving these pa...