The Lange colloidal gold test is to date a purely empirical test since its mechanism is not yet understood. It has been explained on the assumption that the precipitation of the gold sol is due to the discharge of its negatively charged particles by the positively charged protein ions of the spinal fluid. These positively charged ions are so charged because the proteins from which they are derived are on the acid side of their iso-electric point. This of course presupposes that in a spinal fluid with a positive reaction there is a protein that is on the acid side of its iso-electric point at the pH to which the colloidal gold is adjusted in the Lange test. This conception includes the possibility that there may be present proteins which are on the basic side of their iso-electric point and which, if effective, act as protective substances. The hypothesis