Intrinsically disordered proteins such as FlgM play important roles in biology, but little is known about their structure in cells. We used NMR to show that FlgM gains structure inside living Escherichia coli cells and under physiologically relevant conditions in vitro, i.e., in solutions containing high concentrations (≥400 g/L) of glucose, bovine serum albumin, or ovalbumin. Structure formation represents solute-induced changes in the equilibrium between the structured and disordered forms of FlgM. The results provide insight into how the environment of intrinsically disordered proteins could dictate their structure and in turn, emphasize the relevance of studying proteins in living cells and in vitro under physiologically realistic conditions. [abstract from Dedmon, M. M., Patel, C. N., Young, G. B., Pielak, G. J. FlgM gains structure in living cells. Proceedings of the National Academy of Sciences of the United States of America 99, 12681-12684 (2002).]