Physicochemical studies have suggested that the 43-kDa plasma membrane fatty acid binding protein (FABPpm) is closely related to the mitochondrial isoform of aspartate aminotransferase (mAspAT). In the present studies, mAspAT was not detected immunohistochemically or by immunoblotting in plasma membranes of proliferating 3T3-L1 fibroblasts. During controlled differentiation to an adipocyte phenotype, mAspAT became detectable by the second day of confluent growth, prior to accumulation of visible lipid droplets, and was strongly expressed in 8-day differentiated 3T3-L1 adipocytes. The pattern of expression paralleled the previously reported expression both of FABPpm and of the Vmax for saturable uptake of long chain free fatty acids. As with anti-FABPpm, antibodies to mAspAT selectively inhibited the uptake of [3H]-oleate in 3T3-L1 adipocytes but not in fibroblasts, while having no effect on uptake of either 2-deoxyglucose or the medium chain fatty acid octanoate. Preabsorption of anti-FABPpm with mAspAT, or of anti-mAspAT with FABPpm, abolished immunopositivity in immunohistochemical and immunoblotting studies, as well as the ability of either antibody to inhibit [3H]-oleate uptake. These studies provide strong biologic evidence for the identity of FABPpm and mAspAT, and for the hypothesis that FABPpm/mAspAT mediates the uptake of long chain free fatty acids.