stable transfections - antibiotic dose response (Jun/14/2002 )

I use the PcDNA 3.1 to transfect the Mc 3T3 E1 osteoblast cell line. before I transfected the cell, I make the kill curve to decide the dose of G418. from the kill curve, I found that 400 to 500 micro gram per ml is suitable. but, the result is that there is very very small cell died while 800 micro gram per ml kill more cells. Anybody knows the reasons?