Twenty-two proteins available to this laboratory were examined for their ability to crystallize from polyethylene glycol (PEG). Of the 22, 8 had been crystallized previously by conventional means and the remainder had yielded no crystals by other methods. Using a single broad screen with four different concentrations of five different molecular weight sizes of polyethylene glycol, 13 of these proteins were crystallized. Of the 13, 6 proteins were crystallized for the first time, one yielded crystals readily where only infrequent and irreproducible results were obtained before, and one other produced excellent quality crystals where only very disordered or twinned crystals were previously grown. In only one case did polyethylene glycol fail to yield crystals where other agents had succeeded. It is concluded that polyethylene glycol may be the best initial trial reagent for crystallization of proteins for x-ray diffraction analyses.