Although virtually all cells store neutral lipids as cytoplasmic lipid droplets, mammary epithelial cells have developed a specialized function to secrete them as milk fat globules. We have used the mammary epithelial cell line HC11 to evaluate the potential connections between the lipid and protein synthetic pathways. We show that unsaturated fatty acids induce a pronounced proliferation of cytoplasmic lipid droplets and stimulate the synthesis of adipose differentiation-related protein. Unexpectedly, the cellular level of beta-casein, accumulated under lactogenic hormone treatment, decreases following treatment of the cells with unsaturated fatty acids. In contrast, saturated fatty acids have no significant effect on either cytoplasmic lipid droplet proliferation or cellular beta-casein levels. We demonstrate that the action of unsaturated fatty acids on the level of beta-casein is post-translational and requires protein synthesis. We have also observed that proteasome inhibitors potentiate beta-casein degradation, indicating that proteasomal activity can destroy some cytosolic protein(s) involved in the process that negatively controls beta-casein levels. Finally, lysosome inhibitors block the effect of unsaturated fatty acids on the cellular level of beta-casein. Our data thus suggest that the degradation of beta-casein occurs via the microautophagic pathway.