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 engra
ftment percentage is seen, but when the same total number of cells is
given over five injections (separated by 24 hours), the observed engra
ftment is much higher. A further increase in engraftment appears to oc
cur 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 an
d that the keys to high levels of engraftment are the cell cycle statu
s of the engrafting stem cell and the schedule of engraftment.