This study is concerned with the influence of biological suspension on the position of the binodal curve in aqueous two-phase systems (ATPSs). Three different biological suspensions (i.e., disrupted yeast, E. coli homogenate and fermentation broth from Trichoderma harzianum) were selected and their impact upon ATPS performance was evaluated on the basis of changing volume ratio (V(r)) and the position of the binodal curve of biological ATPSs (added with biomass). Biological ATPSs with initial V(r) greater than 1 and long (>40%, w/w) tie-line length (TLL), exhibited significant changes in V(r) when compared with that from non-biological systems. Such behaviour was associated with the top phase biomass accumulation. It was shown that the addition of the biological suspensions used in this study to ATPSs caused the binodal curve to displace towards the origin, which was associated with the critical contribution of the bio-polymer (present in the systems) to the phase formation. The practical implementation of ATPSs for the purification of biological materials exploiting the information reported in this study is discussed. (C) 2000 Elsevier Science B.V.