The conformational flexibility of FtsZ and the properties of its epitopes have been studied. Cellular fractions of Escherichia coli have been treated with Triton X-114. FtsZ distributed in the polar as well as in the non-polar phase. This has been interpreted to mean that FtsZ can change its conformation. For the nonpolar conformation it has been assumed that the putative hydrophobic pocket of FtsZ (cf. Voskuil et al., J. Bacteriol. 176:1886-1893) is being turned inside out upon interaction with the cytoplasmic membrane. In a tentative model we suggest that FtsA mediates this interaction. Immunoprecipitations of FtsZ with various monoclonal antibodies in the presence or absence of 1 M NaCl gave a clue concerning the hydrophobicity and hydrophilicity of FtsZ's epitopes. Immunogold-labeling also showed differences with respect to the accessibility of FtsZ.