Trypsin-modified elongation factor (EF-)Tu-GDP from Escherichia coli is known to crystallize in several different unit cells under apparently identical conditions. The crystal polymorphism was investigated and found to be correlated with the source of polyethylene glycol used in the crystallization procedure. The use of highly purified polyethylene glycol promoted the growth of a new crystal form belonging to space group P2(1)2(1)2(1) with cell dimensions of a = 71.9 A, b = 74.7 A, c = 170.9 A and two molecules per asymmetric unit. In extensive crystallization trials, substances that typically contaminate commercial preparations of polyethylene glycol were screened. The final results show that the presence of the divalent anions, HPO4(2-) or SO4(2-), at different concentrations induce the growth of two known crystal forms belonging to space groups C222(1) and P4(3)2(1)2. The relevancy of the findings is discussed.