In this study we have investigated the use of flotation and filtration, singly and combined, to enhance the separation of plasmid containing liquors from neutralised lysates with very different levels of solids. Filtration of crude neutralised lysates, containing roughly 100 g l−1 solids, through various diatomaceous earth and cellulose precoat materials was invariably accompanied by severe loss of plasmid through adsorption and/or absorption. The use of more refined and inert filter aids did not alleviate these problems. The finest filter aid, Celatom FP-1SL, gave the best compromise of filtrate clarity (solids content of 0.05 g l−1) and plasmid purity (71%) and was selected for further studies involving combined use of flotation and filtration. Removing the vast bulk of solids prior to filtration by flotation of the floc and draining of the plasmid liquor beneath, impacted dramatically on the filtration performance. Though systematic reductions in the solids challenge per unit filter area were accompanied by increased flux, elevated levels of solids extrusion, chromosomal DNA and protein contamination were also observed, and losses of plasmid to filter aids were still high. We have observed that increasing the scale of operation during lysis and neutralisation from 0.3 or 0.6 l to 15 l is accompanied by significant improvements in separation of cell debris solids from the plasmid and increased recoveries of the plasmid containing liquor. At the latter scale, the drained liquor contained ∼80% of the plasmid and the solids content was only 0.2 g l−1.