A new method for the determination of rubin numbers has been advanced. The previous method based on the measurement of the blue colour was discarded as the blue dye was found to be in suspension. The rubin number of a protective colloid is defined as the number of milligrams of that colloid which when present in 10 c.c. of 0·002% congo rubin solution of pH 5·1 produces 9·0 Lovibond units of red colour in a 0·6″ cell on addition of 1 c.c. of 10% sodium chloride and centrifuging. The results obtained by this method in the case of starch, casein and albumin show the same trend as the results of the earlier methods.