question about the protein extraction - (Mar/23/2005 )

hi, everyone:
I meet a problem confusing me very much. I use the same amount of cells to do transfection and then extract the protein from cell lysate. The first time I can get more than 100ug protein but the second and third time I can only get 5oug or even less. The procedure to extract protien does not change at all. Could you help me to analyze it?

Thanks.

Maybe you didn't disrupt the cells fully.
Is everything the same: same cells, same plasmid, same temp, same aeration, same concentrations, same brands of chemicals?
Expression does vary, although it is more like 6mg/l to 12mg/l with my large scale overexpression, sometimes it is luck.