Flow cytometry has been used to study the contents of macromolecular compounds and light-scatter parameters in batch and continuous cultures of a recombinant Escherichia coli strain that forms protein inclusion bodies. Changes in relative DNA and RNA contents and cell mass as estimated by forward-angle light scatter were detected and tightly correlated in batch culture. In addition, heterogeneity of wide-angle light scatter (WALS), which we related to the presence of cellular inclusion bodies, was observed. In contrast, the relative RNA content and cell mass did not change during continuous culture, and homogeneity of WALS was found. In addition, unexpected changes in relative DNA content were observed after 67 h of culture, indicating a change in bacterial physiology.