W/Wv mice with congenitally defective CFU proliferation and their normal, congenic littermates were used as hosts for diffusion chamber (DC) implants. CFU growth in implanted allogenic CF1, or congenic +/+ marrow was significantly greater in W/Wv than in control hosts. When W/Wv mice were “cured” of their hemopoietic defect, CFU proliferation in the DCs decreased, but not to the control level. These observations have provided evidence for humoral control of CFU growth related to a genetic stem cell defect. Diffusion chamber myelopoiesis was also enhanced in W/Wv hosts. In comparison with their congenic controls, W/Wv mice were neutropenic and had decreased numbers of marrow myeloid elements. Thus, a humorally mediated feedback related to a defective myelopoiesis in the hosts might have accounted for increased DC myelopoiesis. However, a “spillover”effect from increased stem cell growth has not been excluded.

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