Helen Assuming you have >10 7 individual plaques, I would recommend storing the libraries in 10 aliquots. 9 at -70 in dmso and 1 at 4o (+ CHCl3) as your working stock. When that is used up, thaw another aliquot. Having said that, I have freeze-thawed such libraries 2 or 3 times without detectable loss of titre. John