The long-range goal of this program is to define the immunobiology of mononuclear phagocy te (MNP) ontogeny. The first phase of this program will focus on the hypothesis that macrophage functional heterogeneity may, in part, be due to the existence of multiple macrophage cell lineages. We will test this hypothesis using murine Ia+ bone marrow-(BM) derived MNP which appear to arise from a progenitor distinct from that yielding the Ia-BM MNP. In addition to determining phenotype stability and associating selected immunological functions with surface phenotype, plans are proposed to characterize established cell lines of MNP.