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Careina, One thing to try if other ideas don't work or are too difficult, is covalently (therefore unambiguously) labelling a little of your protein with a fluorescent dye. If you add 20 nL of this to the drop *after the crystals have grown*, protein crystals will light up, but salt crystals will not. Thermo make some very easy-to-use kits for labelling. See methods section of our paper *Cryst. Growth Des.*, 2011, *11* (8), pp 3432–3441. Could you also label the DNA . . . ? Hope it helps, best wishes, Patrick On 15 April 2013 11:18, Careina Edgooms <[log in to unmask]> wrote: > Dear ccp4 > > I have been performing trials on a protein DNA complex for a while now and > have not seen any crystals form. Today I checked an old plate (over a month > old) and I see 4 large crystals. *excitement* Three of them look tetragonal > in shape (like a pyramid) and one of them looks hexagonal. I do not know if > they are salt or protein. There is calcium chloride in the buffer. They > feel quite soft to touch. They do not cause much birefringence. One of them > does not seem to absorb much izit. It did go a bit blue but not entirely. > > How can I tell if this crystal is protein or not? Do you think its worth > trying to see how it diffracts? > > Also, does Izit affect diffraction/ protein structures at all? Could I use > a crystal with Izit in a diffraction experiment and ultimately to get the > structure? > > Best > Careina > -- [log in to unmask] Douglas Instruments Ltd. Douglas House, East Garston, Hungerford, Berkshire, RG17 7HD, UK Directors: Peter Baldock, Patrick Shaw Stewart http://www.douglas.co.uk Tel: 44 (0) 148-864-9090 US toll-free 1-877-225-2034 Regd. England 2177994, VAT Reg. GB 480 7371 36