The c-Src tyrosine kinase phosphorylates and binds to a 68-kDa RNA-binding protein in mitotic cells. We have examined the mechanism and functional consequence of the interaction of c-Src with this protein, Sam 68 (Src associated in mitosis, 68 kDa). In whole cell homogenates, Sam 68 was the predominant substrate and binding partner of overexpressed c-Src. Mitotic, tyrosine-phosphorylated Sam 68 bound selectively to recombinant SH2 domains with significantly different affinities (c-Src approximately Ras GTPase activating protein > p85 alpha (amino-terminal) > Grb2 >> p85 alpha (COOH-terminal)). In vitro translated Sam 68 also bound selectively to recombinant SH3 domains, with the highest affinity for the Src and p85 alpha SH3 domains. SH3 binding was inhibited by specific Sam 68 peptides. In vitro translated Sam 68 bound directly to immobilized poly(U), and this was inhibited by binding of Src and p85 SH3 domains to Sam 68. The results suggest that the selection of Sam 68 as a mitotic target by c-Src is the result of highly specific interaction with SH2 and SH3 domains and that this interaction may modulate the RNA binding activity of Sam 68.