Rats were injected intravenously with various particulate materials with and without the addition of various stabilizing agents. One hour after the injection of the blockading colloid, a tracer dose of similar or different colloidal material was injected. A half-time clearance rate of the tracer dose greater than 4 minutes was taken as evidence of blockade of the RES.

Blockade of the tracer dose occurred when the surface properties of the particles in both blockading and tracer doses were identical. Different particles stabilized by the same agent behaved as identical particles and identical particles stabilized by different agents behaved as different particles.

The opsonization of a tracer dose of gelatin-stabilized colloid in vitro by a specific globulin fraction obtained from heterologous and homologous plasma prevented its blockade, while opsonization of both blockading and tracer doses with the same proteins resulted in blockade. However, when both blockading and tracer doses were opsonized with isologous globulins, blockade did not occur.

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