Blk, also known as B lymphoid kinase, is a 55 kd tyrosine kinases with a SH3, SH2 and catalytic domains that contain consensus sequences of the src protein tyrosine kinase family. Blk is expressed specifically in the B cell lineage and plays a role in signal transduction pathway that is restricted to B lymphoid cells (1). Stimulation of resting B-lymphocytes with antibodies to surface immunoglobulin (sIgD or sIgM) induces activation of Blk (2). Upon activation, the SH2 domain of Blk binds more than 10 distinct phosphoprotein including those of 72, 76, 115, and 130 kd (3). Blk prefers peptide substrates of the structure I/L-Y-D/E-X-L which resemble critical features of the ITAM motifs found in, e.g. the intracellular components Ig-α an