Membrane-associated adaptors play an important role in coupling antigen receptor engagement to downstream signaling events, such as Ras-MAPK activation, Ca(2+) flux, and nuclear factor of activated T cells (NFAT) activation. Here we identified a novel membrane-associated adaptor protein, LAX. LAX is mainly expressed in B cells, T cells, and other lymphoid-specific cell types. It shares no overall sequence homology with LAT and is not localized to lipid rafts. However, like LAT, LAX has tyrosine motifs for binding Grb2, Gads, and the p85 subunit of phosphatidylinositol 3-kinase. Upon stimulation via the B or T cell receptors, LAX is rapidly phosphorylated by Src and Syk family tyrosine kinases and interacts with Grb2, Gads, and p85. Overexpression of LAX in Jurkat cells specifically inhibits T cell receptor-mediated p38 MAPK activation and NFAT/AP-1 transcriptional activation. Our data suggested that LAX functions to negatively regulate signaling in lymphocytes.