Adipocyte differentiation-related protein (ADrP) is an intrinsic lipid storage droplet protein that is highly expressed in lung. ADrP localizes to lipid storage droplets within lipofibroblasts, pulmonary cells characterized by high triacylglycerol, which is a precursor for surfactant phospholipid synthesis by alveolar type II epithelial (EPII) cells. The developmental pattern of ADrP mRNA and protein expression in lung tissue parallels triacylglycerol accumulation in rat lung. ADrP mRNA levels are relatively high in isolated lipofibroblasts, accounting for the high ADrP expression in lung. Isolated EPII cells, which do not store neutral lipids but derive them from lipofibroblasts, have low levels of ADrP mRNA expression. ADrP is found around lipid droplets in cultured lipofibroblasts, but not in EPII cells isolated from developing rat lung. After coculture with lipofibroblasts, EPII cells acquired ADrP, which associates with lipid droplets. Furthermore,³H-labeled triolein in isolated ADrP-coated lipid droplets is a tenfold better substrate for surfactant phospholipid synthesis by cultured EPII cells than ³H-labeled synthetic triolein alone. Antibodies to ADrP block transfer of neutral lipid. These data suggest a role for ADrP in this novel mechanism for the transfer of lipid between lipofibroblasts and EPII cells.