pp72 syk is essential for development and function of several hematopoietic cells, and it becomes activated through tandem SH2 interaction with ITAM motifs in immune response receptors. Since Syk is also activated through integrins, which do not contain ITAMs, a CHO cell model system was used to study Syk activation by the platelet integrin, α IIb β 3. As in platelets, Syk underwent tyrosine phosphorylation and activation during CHO cell adhesion to α IIb β 3 ligands, including fibrinogen. This involved Syk autophosphorylation and the tyrosine kinase activity of Src, and it exhibited two novel features. Firstly, unlike α IIb β 3-mediated activation of pp125 FAK, Syk activation could be triggered by the binding of soluble fibrinogen and abolished by truncation of the α IIb or β 3 cytoplasmic tail, and it was resistant to inhibition by cytochalasin D. Secondly, it did not require phosphorylated ITAMs since it was unaffected by disruption of an ITAM-interaction motif in the SH2 (C) domain of Syk or by simultaneous overexpression of the tandem SH2 domains. These studies demonstrate that Syk is a proximal component in α IIb β 3 signaling and is regulated as a consequence of intimate functional relationships with the α IIb β 3 cytoplasmic tails and with Src or a closely related kinase. Furthermore, there are fundamental differences in the activation of Syk by α IIb β 3 and immune response receptors, suggesting a unique role for integrins in Syk function.