beta-Galactosidase was induced in a wild-type and an rpsL mutant E. coli at 37 and 42 degrees C (heat shock). The samples were subjected to capillary electrophoresis-based single enzyme molecule assays and the distribution of the populations' catalytic rates and electrophoretic mobilities were determined. The populations were found to differ significantly with respect to both properties which is consistent with conformational differences between molecules being an underlying basis for enzyme heterogeneity. The samples were also subjected to partial thermal denaturation which, in all but one population, led to a decrease in average activity and a decrease in variance. Furthermore, it was found that the electrophoretic mobility of the enzyme was altered by the presence of citrate in the buffer system, likely due to nonspecific binding.