Differentiation of ST-13 preadipocytes into adipocytes was inhibited almost completely by addition of rat plasma fibronectin (FN) (approximately 100 micrograms/mL), but was reversed by GRGDSP cell recognition peptide (1.5 mM) and anti-alpha 5 beta 1. On the contrary, the thermolysin digest of FN stimulated adipocyte differentiation in a dose-dependent manner, in which remarkable increases in the values of the differentiation indexes, the number of adipocytes (8-fold above the control), glycerophosphate dehydrogenase (GPD) activity (12-fold), and triacylglycerol content (5-fold), were observed by inclusion of the thermolysin digest (100 micrograms/mL). The increase in GPD activity by the thermolysin digest was inhibited remarkably (about 70% inhibition) by an antibody directed to the amino-terminal fibrin-binding (Fib 1) domain of FN and slightly (about 15%) by an antibody directed to the central cell-binding (Cell) domain, but not by anti-gelatin-binding domain and anti-carboxy-terminal fibrin-binding domain. Treatment of ST-13 cells by a purified 24K fragment (100 micrograms/mL) derived from the Fib 1 domain caused an over 20-fold augmentation of the GPD activity, accounting for a major part of the differentiation stimulatory activity of the thermolysin digest. The differentiation stimulatory effect of the 24K Fib 1 fragment was not affected by either GRGDSP peptide or anti-alpha 5 beta 1. Thus, FN can regulate adipose development of ST-13 cells by its two antipodal, inhibitory and stimulatory, activities, the latter of which is expressed only upon fragmentation. Proteolytic cleavage of FN may play an important role in controlling the action of FN on adipocyte differentiation.