Based on sequence relationships the cytoplasmic fatty acid-binding proteins (FABPs) of mammalian origin are divided into at least three distinct types, namely the hepatic-, intestinal- and cardiac-type. Highly conserved sequences of FABPs within the same type correlate with immunological crossreactivities. Isoforms of hepatic-type FABP are found in several mammalian species and for bovine liver FABP specific shifts in isoelectric points upon lipidation with fatty acids are observed. Isoforms of intestinal-type FABP are not known and the occurrence of cardiac-type isoforms so far is confined to bovine heart tissue. A bovine mammary-derived growth inhibitor (MDGI) is 95% homologous to the cardiac-type FABP from bovine heart. Dissociation constants of FABP/fatty acid complexes are in the range of 1 microM and 1:1 stoichiometries are usually found, but the neutral isoform of hepatic FABP from bovine liver binds 2 fatty acids. On subcellular levels hepatic- and cardiac-type FABPs are differently distributed. Though mainly cytosolic in either case, immunoelectron microscopy as well as a gelchromatographic immunofluorescence assay demonstrate the association of hepatic FABP in liver cells with microsomal and outer mitochondrial membranes and with nuclei, whereas in heart cells cardiac FABP is confined to mitochondrial matrix and nuclei. In mammary epithelial cells MDGI is associated with neither mitochondria nor endoplasmic reticulum, and is expressed in a strictly developmental-dependent spatial and temporal pattern. The specific role proposed for MDGI is to arrest growth of mammary epithelial cells when they become committed to differentiation in the mammary gland.