The uptake of heat-denatured BSA (c BSA) by these cells in vivo is 3.5 µg/mg c BSA injected/10⁸ cells. The uptake by cells in animals which were exposed 2 days earlier to 900 r whole body irradiation is slightly lower but does not differ significantly. 90% of the phagocytosed material is degraded within 2–3 hr, the residual 10% is retained at least over an 8 hr periods. Using a pulse and chase technique, with ¹²⁵I and ¹³¹I c BSA in vitro and in vivo it was shown that newly phagocytosed antigen is found mainly in a lysosomal turnover compartment of a density 1.19 g cm^–3. Antigen which has been in the cells for longer was found in a denser fraction (1.26 g cm^–3). In a comparison of nhrmal and X-irradiated cells it can be shown that after irradiation with 900 r less c BSA is found in this storage compartment.

Binding of the antigen to the subcellular fractions, and its behavior towards several detergents has been studied. Subcellular fractions do not have the increased immunogenic capacity of antigen enclosed in living macrophages.

Two synthetic polypeptide antigens, poly(D-Tyr, D-Glu, D-Ala) and poly-(L-Tyr, L-Glu) have a different subcellular distribution from c BSA, BSA, or bovine gamma globulin. Apart from also being found in the 1.26 storage compartment the polypeptide antigens are mainly located in a 1.15 compartment and only to a small extent in the 1.19 compartment. The half-life of these antigens in the cells is much longer than the half-life of the protein antigens studied.

The finding of several subcellular compartments is discussed in connection with the functions possibly performed by macrophages.