Acidic and basic fibroblast growth factors (FGF-1 and FGF-2) are strong mitogens for many tumor cell types and potent inducers of angiogenesis in vitro and in vivo. It is notable that these proteins lack classical signal sequences for secretion; the mechanism by which they are released from cells remains obscure. We demonstrate here that FGF-1 is constitutively exported by tumor cell lines derived from highly angiogenic beta cell tumors of transgenic mice. Remarkably, FGF-1 is sequestered as a latent form in the conditioned medium, as assessed by a lack of mitogenic activity and heparin affinity. High salt treatment of conditioned medium unveils the sequestered FGF-1 as novel high molecular weight forms with reduced heparin affinity and a molecular mass of approximately 30-40kDa; we refer to these as exported forms of FGF-1 (XP-FGF-1). Reducing and denaturing agents convert XP-FGF-1 into the 18kDa monomeric form of FGF-1, indicating it represents tight aggregates of FGF-1. XP-FGF-1 is found in cell lysate as well as conditioned medium, suggesting that they represent export intermediates. Brefeldin A, an inhibitor of conventional secretion, does not interfere with FGF-1 export. Moreover, cell lysis or apoptosis are not involved in this export pathway. Therefore, these data demonstrate that FGF-1 is constitutively exported by beta tumor cell lines via a novel secretory pathway so as to facilitate tumor growth and angiogenesis.