Cholesteryl ester (CE) accumulation in arterial wall macrophages (foam cells) is a prominent feature of atherosclerotic lesions. We have previously shown that murine J774 macrophages, unlike mouse peritoneal macrophages, accumulate large amounts of CE from unmodified low density lipoprotein (LDL). We now report a direct comparison of acyl coenzyme A:cholesterol acyl transferase (ACAT) activity in J774 and mouse peritoneal macrophages. Despite similar chloroquine-inhibitable 125I-LDL degradation in the two macrophages, ACAT activity in LDL-treated J774 macrophages was 10-30-fold higher than that in LDL-treated mouse peritoneal macrophages. In contrast, acetyl-LDL (matched for degradation with LDL) caused marked stimulation of ACAT activity in mouse peritoneal macrophages. From these data we conclude that in the presence of LDL, J774 macrophages have a highly active ACAT cholesterol esterification pathway compared with mouse peritoneal macrophages; and in mouse peritoneal macrophages, there is a marked difference in the ability of acetyl-LDL vs. LDL to stimulate ACAT even when the lipoproteins are matched for degradation.