The continuous in vitro marrow culture system for proliferation of mouse pluripotent hemopoietic stem cells (CFUs) and granulocyte-macrophage progenitor cells (CFUc) as initially described by T. M. Dexter, depended upon a 25% concentration of special lots of horse serum and addition of fresh "recharging" marrow after 3-4 weeks. This system has been modified to permit longer hemopoiesis in non-recharged cultures over 25-30 weeks. Addition of 10(-7)M hydrocortisone sodium hemi-succinate during weekly feeding and switch from horse to 25% fetal calf serum with corticosteroid at week 4, maintains stability of the adherent marrow stromal cells, decreases lipogenesis (which is deleterious after 10 weeks) and increases proliferation of hemopoietic cells over longer duration. Stem cells removed from 8 week old primary NIH/Swiss marrow cultures reconstituted hemopoiesis in lethally irradiated mice. While 16 week old cultures produced total numbers of CFUc equivalent to 8 week cultures, these cells could not prevent marrow death following similar total body irradiation. Thus, stem cells moved progressively from a pluripotent to a committed CFUc compartment between weeks 8-16. Human femur marrow required thyroxine in addition to hydrocortisone for sustained CFUc maintenance over a shorter 8 week period. Improved technology for mouse long-term marrow cultures should further aid in developing a usable human "Dexter-system".