An understanding of blood cell formation, both normal and neoplastic, is dependent on firm knowledge of the dynamics of the stem cell population, from which all circulating cells are ultimately derived. We propose studies of stem cells based on our previous identification of novel phenotypes involving stem cell proliferation and differentiation. We have found that proliferative activity in the stem cell compartment varies among inbred strains of mice and preliminary evidence is consistent with the hypothesis that genetic control resides in a single locus that we know is not Fv-2. In chimeric (allophenic) mice constructed using inbred strains with different stem cell proliferative activities, erythropoiesis is skewed toward the strain characterized by greater stem cell proliferation. We plan to study the interrelationship between these two newly described phenotypes. 1. Using recombinant inbred (RI) mouse strains we plan to determine how many genes are involved in the regulation of strain-dependent stem cell proliferation. By comparing our results with a compiled RI strain distribution pattern for other phenotypes, we will determine if it is linked to any other genes and, if so, which, and where they map. 2. We will use two probes to dissect the stem cell population to determine where the proliferative difference is manifested in the hierarchy of developmentally restricted subpopulations. The first is 5-fluorouracil which selectively spares the most primitive subset and the second is a monoclonal antibody to an alloantigen expressed preferentially on the most primitive stem cells. 3. If stem cell proliferative activity is directly related to accentuated erythropoiesis in allophenic mice it follows that, in chimeras, stem cell populations should behave kinetically as they do in the syngeneic state. We will determine if genotype-specific differences in stem cell proliferation are maintained in the chimeric environment and hence if stem cell populations are autonomously regulated. 4. Our preliminary results show significant differences in hemoglobin synthesis between bone marrow cultures from inbred mouse strains. This work provides an experimental approach that we will use to understand different rates of erythropoiesis in allophenic mice. (M)