We have studied the cause of inclusion body formation in Escherichia coli grown at 37 degrees C using statistical analysis of the composition of 81 proteins that do and do not form inclusion bodies. Six composition derived parameters were used. In declining order of their correlation with inclusion body formation, the parameters are charge average, turn forming residue fraction, cysteine fraction, proline fraction, hydrophilicity, and total number of residues. The correlation with inclusion body formation is strong for the first two parameters but weak for the last four. This correlation can be used to predict the probability that a protein will form inclusion bodies using only the protein's amino acid composition as the basis for the prediction.