Engraftment of normal murine marrow into nonmyeloablated host mice.

When 200 x 10(6) male BALB/c cells are given by tail vein injection to female nonmyeloablated hosts in one injection, a relatively low engraftment percentage is seen, but when the same total number of cells is given over five injections (separated by 24 hours), the observed engraftment is much higher. A further increase in engraftment appears to occur when the same number of cells is given in 10 injections separated by at least 24 hours. These data suggest that somewhere between 5 and 10% of marrow niches are available at intervals of 24 or more hours and that the keys to high levels of engraftment are the cell cycle status of the engrafting stem cell and the schedule of engraftment.