Killer inhibitory receptor (KIR) inhibits cytolytic function of killer cells by specific interaction with class I MHC molecules. The inhibitory effect mediated by KIR requires co-engagement of KIR with an activating receptor, such as TCR or FcR. This implies that KIR may function in the immediate vicinity of activating molecules, and previous studies have shown that p58 KIR is associated with TCR zeta- and FcR gamma-chain in NK cells. To better understand the molecular interaction between KIR and TCR zeta-chain, we generated a His-tag fusion protein of a p70 KIR cytoplasmic tail (His-CytKIR) and used this protein to coprecipitate TCR zeta-chain from Jurkat T cells. Western blots of the resolved coprecipitates showed that the cytoplasmic tail of KIR associates with TCR zeta in vitro. Interestingly, the association between the His-CytKIR and TCR zeta was dependent on the phosphorylation of the His-CytKIR. Unlike the unphosphorylated His-CytKIR, the phosphorylated form no longer associated with TCR zeta. However, the association was not affected by the tyrosine phosphorylation of TCR zeta. These results suggest that the cytoplasmic tail of KIR may couple to TCR zeta in a phosphorylation-dependent manner, so it could fine-tune the activation signals induced via the TCR.