Mononuclear phagocytes and dendritic cells were analyzed in detail with a panel of monoclonal antibodies by immunoalkaline phosphatase and immunofluorescence techniques. The panel included two antibodies (KB90 and SHCL3) against the two chain membrane constituent, P150,95, an antibody (Mo1) against C3bi receptor, an antibody (To5) against C3b receptor, and two antibodies (Mo2 and UCHM1) against a single chain glycoprotein with a molecular weight of 55 to 60 previously found on monocyte/macrophages. In addition, the panel contained several antibodies including EBM11, LeuM3 and FMC32 whose molecular specificities are not known at present. Frozen sections of spleen, bone marrow, thymus, lymph node, liver, brain, lung, kidney, colon, and skin were examined. Several populations of mononuclear phagocytes and dendritic cells could be distinguished on the basis of their reactivity with the antibody panel. Populations of macrophages resident in parenchymal organs had reaction patterns distinct from those of circulating macrophages. Many of the antibodies in the panel reacted with both macrophages and dendritic cells suggesting a close cytogenic, if not functional, relationship between these cells. There was wide variability in the reactions of macrophages and dendritic cells in different tissue and less variability among cells within individual anatomic locations.