Reticuloendothelial clearance of carbon particles is inhibited by injection of heat-aggregated albumin. The possibility that this inhibition might involve a serum component has been explored. Evidence that such a serum factor does participate in carbon clearance inhibition by heat-aggregated albumin derives from the fact that prior treatment of the carbon with homologous serum eliminates the blockade; and also, if serum previously absorbed with carbon is mixed with the carbon particles no restoration of "blockade" by albumin occurs. A method for extracting the component from serum using barium sulfate adsorption is described which allows concentration of the factor and sensitive assay for its presence. It is concluded that while large or multiple carbon injections may bring about a saturation of the phagocytic cell, this saturation masks an underlying factor which protects against acute inhibition of carbon phagocytosis by denatured albumin when subsaturating carbon doses are used.

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