Lipoprotein lipase, a glycoprotein, acts at the luminal surface of capillaries to hydrolyze triacylglycerol of chylomicrons and VLDL. Tissue cells other than endothelium are believed to synthesize the enzyme. Distribution of lipoprotein lipase in mouse heart was studied with an indirect immunocyto-chemical method using chicken antiserum to bovine milk lipoprotein lipase, which cross-reacts with mouse lipoprotein lipase, and gold- or ferritin-labeled second antibodies. This approach demonstrates protein which could be either an active or an inactive form of the enzyme. Gold- and ferritin-immunolabels were found mostly in capillaries, along the luminal surface and associated with chylomicron remnants and endothelial projections, thus locating lipoprotein lipase at sites of activity in capillaries. Gold- and ferritin-immunolabels were also found in myocytes, within endoplasmic reticulum and Golgi cisternae, and over vesicles between Golgi and the plasma. Immunolabel was sometimes found between myocytes and endothelial cells, and within and between endothelial cells. These findings demonstrate that cardiac myocytes contain lipoprotein lipase distributed in organelles that are involved in the synthesis, processing and secretion of glycoproteins. Thus, cardiac myocytes are probably the main source of lipoprotein lipae in capillaries in heart. Mice with Combined Lipase Deficiency (cld/cld) have very low levels of lipoprotein lipase activity in their heart and brown adipose tissue. However, these tissues contain 3-5 X normal amounts of lipoprotein lipase protein. Ferritin-immunolabel was found over large perippheral vacuoles in cardiac myocytes and brown adipocytes in cld/cold mice. These findings suggest that secretion of lipoprotein lipase by cells that synthesize the enzyme is defective in cld/cld mice.