The lipid in milk (milk fat) is found in the form of droplets known as milk lipid globules (MLG). These milk lipid globules are encompassed by a unit membrane known as the milk lipid globule membrane (MLGM) which is derived from the apical plasma membrane of the mammary epithelial cell during secretion. In lactating mammary epithelial cells, immediate precursors of milk lipid globules appear to be cytoplasmic lipid droplets (CLD). These cytoplasmic lipid droplets have diameters >1 μm and are characterized by an electron dense, granular surface coat. A previously unrecognized group of structures with diameters <.5 μm, which resemble cytoplasmic lipid droplets in matrix and surface coat appearance, has been observed. The surface coat of these triacylglycerol containing structures, termed micro lipid droplets (μLD), was similar to that of cytoplasmic lipid droplets in enzyme and polypeptide composition. Morphological evidence suggested that these small structures may originate from rough endoplasmic reticulum (RER) and fuse with cytoplasmic lipid droplets. Immunochemical studies showed homology of certain proteins among the rough endoplasmic reticulum, micro lipid droplets and cytoplasmic lipid droplets, which supported the possibility of an endoplasmic reticulum origin of these droplets. The rate of incorporation of [1-¹⁴C]-palmitate and [1,2,3-³H]-glycerol into lipid of RER, μLD, CLD and MIG fractions suggested a possible translocation pathway of triacylglycerols from the rough endoplasmic reticulum to cytoplasmic lipid droplets. The micro lipid droplets seem to provide triacylglycerols to support growth of cytoplasmic lipid droplets. In addition, morphological evidence suggested that these micro lipid droplets can be secreted directly in a manner similar to cytoplasmic lipid droplets, providing for the small lipid globules in milk. Little is known concerning the biochemical processes of milk lipid secretion but it is thought that butyrophilin, a glycoprotein found in milk lipid globule membrane, may play a role. After treatment of mammary epithelial cells with tunicamycin, butyrophilin content of this membrane is reduced. Thus a method for the study of the physiological role of this glycoprotein is proposed.