AIM/CD69 is the earliest leukocyte activation antigen and is expressed mainly by activated T, B, and natural killer (NK) cells. It is also constitutively expressed by platelets, by bone marrow myeloid precursors, and by small subsets of resident lymphocytes in the secondary lymphoid tissues. The engagement of CD69 by specific antibodies induces intracellular signals, including Ca⁺⁺ flux, cytokine synthesis, and cell proliferation. To investigate the physiological relevance of CD69, we generated mice deficient in CD69 (CD69-/-) by gene targeting in embryonic stem cells. CD69 (-/-) mice showed largely normal hematopoietic cell development and normal T-cell subpopulations in thymus and periphery. Furthermore, studies of negative- and positive-thymocyte selection using a T-cell receptor transgenic model demonstrated that these processes were not altered in CD69 (-/-) mice. In addition, natural killer and cytotoxic T lymphocyte cells from CD69-deficient mice displayed cytotoxic activity similar to that of wild-type mice. Interestingly, B-cell development was affected in the absence of CD69. The B220^hiIgM^neg bone marrow pre-B cell compartment was augmented in CD69 (-/-) mice. In addition, the absence of CD69 led to a slight increase in immunoglobulin (Ig) G2a and IgM responses to immunization with T-dependent and T-independent antigens. Nevertheless, CD69-deficient lymphocytes had a normal proliferative response to different T-cell and B-cell stimuli. Together, these observations indicate that CD69 plays a role in B-cell development and suggest that the putative stimulatory activity of this molecule on bone marrow-derived cells may be replaced in vivo by other signal transducing receptors.