An in vivo assay to quantify simultaneously two important components of hepatic reticuloendothelial system (RES) function--clearance and phagocytic killing--was developed in the rat. Intravenously injected E. coli labeled with Na51Cr and 5-[125I]-iodo-2'-deoxyuridine were cleared rapidly from the blood primarily by the liver. While hepatic 51Cr levels remained stable for 24 h following inoculation and provided a reliable measurement of clearance, hepatic 125I decreased over time. Hepatic 125I, calculated by sampling whole liver homogenates, accurately reflects the number of viable bacteria recovered from quantitative cultures of the same homogenates, thus validating this assay as a measure of bacteria killing. Pre-treatment of rats with substances previously shown to affect RES function (gadolinium, zymosan, and sheep erythrocytes) were found to selectively modulate clearance and/or killing. The correlation between hepatic isotope levels and viable hepatic bacteria, the gold standard for assessing the capacity of the liver to remove organisms from the blood and kill them, was preserved under conditions of up- and down-regulation of RES function. The ability to quantitatively discriminate two distinct components of the hepatic RES should provide a useful tool for future investigations of altered RES function.