We have isolated a novel gene, HS1BP3, which encodes an HS1 binding protein. Analysis of HS1BP3 cDNA indicates several potentially important segments, including a PX domain, a leucine zipper, immunoreceptor tyrosine-based inhibitory motif-like motifs and proline-rich regions. HS1BP3 associates with HS1 proteins in vivo as confirmed by immunoprecipitation in B and T cell lines. HS1BP3 preferentially associates with the HS1 SH3 domains rather than with other SH3 molecules, suggesting a role of HS1BP3 as an HS1 signaling mediator. Overexpression of mutant HS1BP3 protein in T cell lines results in decreased IL-2 production. Our data suggest a novel role for HS1BP3 in lymphocyte activation.