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Dear CCP4bb users, I would like to thank everyone who answered my question and apologize for my delay in posting the summary. Here is the question and the answers I got. What was the minimun protein concentration reported with success in crystallization trials? We've done crystallization at 1.2 mg/ml (which was pushing it for that protein), and in some cases, depending on protein solubility of course, we are required to use less than that. We have a protein that (at the moment) precipitates at under 1mg/ml. Gerald Audette I have one with 1.8 mg/ml but this was not due to solubility issues but more not enough protein and desperation mode to set up trays :-) Jürgen Bosch We have at least 1 example where the protein (a large kinase) crystallizes at 2-3 mg/ml but you need seeding Alexander Pautsch I have a database of crystallization conditions extracted from the PDB. I was able to parse the data to extract the concentration of protein in around 900 cases. The lowest 9 protein concentrations were as follows: PDB ID TYPE DATE Protein conc, mg/ml 1EYM ISOMERASE. 07/05/2000 0.75 1ADQ COMPLEX (IMMUNOGLOBULIN/AUTOANTIGEN) 18/02/1997 1 1W5G FOUR HELIX BUNDLE. 06/08/2004 1 1UU4 HYDROLASE. 15/12/2003 1 1W2U HYDROLASE. 08/07/2004 1 1DXP SERINE PROTEASE. 13/01/2000 1 1DY8 SERINE PROTEASE. 18/01/2000 1 1DY9 SERINE PROTEASE. 31/01/2000 1 1DG6 APOPTOSIS. 23/11/1999 1.2 The maximum protein conc was 200 mg/ml (1BLF) My database goes up to October 2004. Janet Newman and Tom Peat have done this job far more conscientiously and could probably give you better and more up-to-date data. Patrick Shaw Stewart Just to add that in one case we were able to get suitable crystals after using a protein concentration of as low as 1.5 mg/ml (kapetaniou et al. Acta Cryst. 2005, F61, 479-481) and microdialysis to slowly remove the excess of salt. Lower concentrations (0.50-0.75 mg/ml) could also produce crystals but they were too small. Tassos Papageorgiou We have one protein (bacterial antitoxin HigA, 2ICT) that crystallizes in many conditions (30-40) at 0.3-0.6 mg/ml; the crystals diffract to 1.63 A. Mark Arbing Once again, thank you for your answers. Ronaldo.