CD200 is a transmembrane molecule with an important immunoregulatory role that is overexpressed on most chronic lymphocytic leukemia (CLL) cells. In this study, we characterized a previously unknown soluble form of this molecule in human plasma termed sCD200. Levels of sCD200 were elevated in the plasma of patients with CLL as compared with healthy controls, and there was a significant correlation with CLL disease stage. Infusion of sCD200^hi CLL plasma into severely immunocompromised NOD.SCIDγ_c^null (NSG) mice enhanced the engraftment of CLL splenocytes as compared with mice receiving sCD200^lo normal plasma. CLL cells were detected in both the spleen and peritoneal cavity of animals for up to 75 days. Engraftment of CLL cells did not occur after infusion of CLL plasma depleted of sCD200 and was abolished in mice treated with anti-CD200 or OKT3 monoclonal antibody (mAb), suggesting a role for both sCD200 and T cells in CLL engraftment. Notably, anti-CD200 mAb was as effective as rituximab in eliminating engrafted CLL cells when administered 21 days after engraftment. Taken together, our findings point to sCD200 as a novel prognostic marker and therapeutic target for CLL. Furthermore, the humanized mouse model described here may prove valuable to preclinically assess new treatment regimens for CLL. Cancer Res; 72(19); 4931–43. ©2012 AACR.