The uptake of radioiodinated mouse and human serum albumin (HSA) by macrophages from non-immunized CBA mice depended on the degree of aggregation of the albumin, and not on cytophilic antibody on the surface of the cell. Autologous and heterologous albumin of comparable physical states were taken up and metabolized equally by macrophages. The HSA taken up by macrophages was rapidly catabolized, except for a small amount that remained associated with the cell both intracellularly and on the

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... ce for long periods in culture. The membrane-bound albumin represented non-pinocytosed molecules. This membrane-bound antigen was maintained independently of intracellular material and was responsible for most of the immunogenicity of macrophage-bound albumin. A portion of albumin taken up by macrophages was released undegraded into the supernatant during culture. The metabolism of HSA was directly proportional to the degree of aggregation of the albumin as increased aggregation resulted in more extensive catabolism.