Fatty acid binding proteins (FABPs) comprise a well-established family of cytoplasmic hydrophobic ligand binding proteins and are thought to be involved in lipid metabolism by binding and intracellular transport of long-chain fatty acids. However, from other studies role for FABPs in cell signalling, growth inhibition and differentiation has also been implied. In particular, the heart-type (H-FABP) is abundantly expressed in differentiated mammary gland and its relationship with a very homologous (95%) mammary derived growth inhibitor (MDGI) was disputed. Here we give a survey on the experimental evidence for the existence of such protein with growth inhibitory function. After cloning of the bovine adipocyte-type (A-)FABP cDNA from mammary gland we conclude that the reported MDGI sequence actually represents a mixture of bovine H- and A-FABP and that the MDGI function is exerted by H-FABP. We also monitored the H-FABP level during differentiation of C2C12 muscle cells from myoblasts to multiply nucleated myotubes. H-FABP expression is clearly detected after that of the transcription factor myogenin which is upregulated immediately upon onset of differentiation and after that of the typical muscle enzyme creatine kinase. This argues against an active role of H-FABP in muscle development unlike the situation in the mammary gland.