AbstractTo study the effect of a perfluorocarbon oxygen transport emulsion (Fluosol‐DA) on reticuloendothelial system (RES) function, we measured the blood clearance of human erythrocytes transfused to rats. Compared with saline treatment, Fluosol‐DA at 30 ml/kg doses significantly increased both the percent 20‐hour blood recovery (mean 8.9% ± 2.7 SEM vs 1.3% ± 0.25 SEM) and51Cr t1/2 survival (mean 14.0 hours ± 2.7 SEM vs 3.5 hours ± 0.33 SEM) of the human red cells. This suppression of RES clearance function was transient and no longer detectable seven days after single Fluosol‐DA doses. The Fluosol‐DA‐induced RES block was about three times greater than that obtainable with 4 g/kg of a soybean oil emulsion used for clinical hyperalimentation. On the other hand, the effect of ethyl palmitate (0.5 g/kg), a potent but toxic RES blocker, was 3.5 times greater than that of Fluosol‐DA in this test system. If Fluosol‐DA also induces RES block in humans, this emulsion could be explored as a therapeutic RES blocker in certai