The regulation of cell surface fibroblast growth factor (FGF) receptors during the differentiation of F9 teratocarcinoma cells was investigated. The capacity of F9 cells to bind '251-basic FGF (FGF-2) increased upon induction of differentiation with dibutyryl CAMP and retinoic acid. No change in binding capacity was observed in the first 24 h after addition of differentiating agents, but a sixfold increase in binding capacity was observed after 48 h and a fivefold increase after 72 h. Scatchard analysis of the binding data indicated that the increased binding of '251-FGF-2 was due to an increase in the number of receptors with no change in their affinity. When '"I-FGF-2 was cross-linked to cell surface receptors, an increase in FGF-2-receptor complexes with molecular weights of 140,00& 160,000 was also observed in the differentiated F9 cells. Undifferentiated F9 cells are known to secrete FGF-4 and cease expression of this molecule upon differentiation. To determine whether the low level of receptors in undifferentiated cells might be related to their production of FCF ligands, the ability of suramin, a drug that can disrupt FGF-receptor interactions, to modulate receptor number on F9 cells was investigated. Surarnin treatment increased the '251-FGF-2 binding capacity of undifferentiated F9 cells threefold but had littleeffect on the binding capacity of differentiated cells. In addition, antibodies to FGF-4 increased the '251-FGF-2 binding capacity of undifferentiated F9 cells by 58%. These results suggest that undifferentiated F9 cells might be responding in an autocrine manner to their own FGF ligands resulting in downregulation of cell surface FGF receptors. The increased number of receptors observed in differentiated cells may partly result from the decreased production of FGF ligands by these cells. o 1994 Witey-Ltss, Inc.