Many soluble proteins crystallize in conditions where their second virial coefficient takes on slightly negative values, known as the crystallization slot, allowing the possibility to screen for crystallizability by measuring virial coefficients. Here we measure virial coefficients for the membrane protein CLC-ecl, which has been crystallized. These virial coefficients reveal that CLC-ecl crystallization conditions do not correspond to the crystallization slot found for soluble proteins and certain membrane proteins. Additionally, while some membrane proteins crystallize under conditions that are associated with a phase transition of the solubilizing detergent and with attractive detergent interactions, it is demonstrated that this is not the case for CLC-ecl crystallization. As with many soluble and membrane proteins, CLC-ecl has been crystallized by adding the neutral polymer poly(ethylene glycol) (PEG). It is shown that the addition of PEG destabilizes detergent micelles and induces CLC-ecl aggregation in crystallization conditions. (C) 2007 Elsevier B.V. All rights reserved.