The disruption of recombinant E. coil cells containing a 76.8 kb plasmid DNA was achieved by the chemical lysis method in a coaxial cylinder rheometer which allowed in situ measurements of rheological changes to be carried out as the lysis reaction proceeded. For the cases studied the cell lysis time was found to be approximately 30 s. Moreover, the release of intracellular material produced a mixture with shear thinning flow properties, the extent of non-Newtonian flow was found to depend on the shear rate used during the cell lysis operation. On neutralisation, the lysate produced a highly flocculated and shear sensitive gel which floated on the top of the liquor containing the plasmid DNA. Small amplitude oscillatory data were obtained showing the viscoelastic properties of the gel matrix. Experimental data were also obtained on the shear sensitivity of the plasmid DNA recovered using a purpose-built rotating disk shear device. Shear rates of the order of 10(6) s(-1) were generated in the device and were confirmed by CFD analysis of the prevailing flow field. Tests carried out with 20 and 29 kb plasmid DNA showed that both plasmids were susceptible to shear damage. The extent of shear damage increased with plasmid size and as the ionic strength of solution decreased.