Recent data suggest that CD44 may serve as a new therapeutic target in AML and possibly also in other myeloid neoplasms. Systemic mastocytosis (SM) is a myeloid neoplasm characterized by abnormal growth and accumulation of mast cells (MC) in one or multiple organs. We have previously shown that normal tissue MC express CD44. In the present study, we asked whether CD44 is expressed on neoplastic human MC and whether CD44-ligation by the monoclonal antibody (mAb) A3D8 would be associated with inhibition of growth of neoplastic MC. As assessed by flow cytometry, primary neoplastic MC were found to express CD44 in all patients with SM analyzed (n=10). The human mast cell leukemia (MCL) cell line HMC-1 was also found to express CD44. As assessed by 3H-thymidine incorporation, the CD44 mAb A3D8 decreased the proliferation of HMC-1 cells in a dose-dependent manner (A3D8, 5 μg/ml: 46±26% of control=100%, p<0.05). Similar effects of A3D8 were observed with primary neoplastic MC obtained from a patient with MCL (A3D8, 5 μg/ml: 68±20%) and one with smouldering SM (A3D8, 2.5 μg/ml: 42±7% of control, p<0.05). To analyze the mechanism of A3D8-induced growth inhibition, cell survival and cell cycle distribution were analyzed. We found that CD44-ligation induces an approximately 3-fold increase in apoptotic HMC-1 cells compared to control. As assessed by flow cytometry, we were also able to demonstrate that A3D8 induces cell cycle arrest in the G1-phase. Moreover, as assessed by Western blotting, we found that incubation of HMC-1 cells with mAb A3D8 results in an increase in the cyclin-dependent kinase-inhibitor p27Kip1. In summary, our results suggest that CD44-ligation is followed by inhibition of growth of neoplastic human MC through induction of apoptosis and G1 cell cycle arrest. The latter is probably mediated by stabilization of p27Kipl. Whether targeting of CD44 in neoplastic MC in patients with high grade MC disorders is of clinical significance remains to be determined in forthcoming studies.