The recently cloned, hemopoietic-specific, src homology 2 (SH2)-containing inositol phosphatase, SHIP, is rapidly gaining prominence as a potential regulator of all phosphatidylinositol (PI)-3 kinase mediated events since it has been shown both in vitro and in vivo to hydrolyze the 5' phosphate from phosphatidylinositol-3,4,5-trisphosphate (PI-3,4,5-P-3). Thus SHIP, and its more widely expressed counterpart, SHIP2, could play a central role in determining PI-3,4,5-P-3 and PI-3,4-P-2 levels in many cell types. To explore the in vivo function of SHIP further we recently generated a SHIP knock out mouse and in this review we discuss experiments carried out with bone marrow derived mast cells (BMMCs) from these animals. (C) 1999 Elsevier Science Ltd. All rights reserved.