The inclusion complexation behavior of paclitaxel with a series of oligo(ethylenediamino) bridged bis(beta-cyclodextrin)s possessing bridge chains in different length (1-4) has been investigated in order to improve the water solubility of paclitaxel. It is found that only the long-tethered bis(beta-cyclodextrin)s 1 and 2 can form the inclusion complexes with paclitaxel, which are characterized by NMR, SEM, XRD, FT-IR, TG-DTA, DSC, and microcalorimetry technology. The results obtained show that bis(beta-cyclodextrin)s 1 and 2 are able to solubilize paclitaxel to high levels up to 2 and 0.9 mg/mL, respectively. The high complex stability of bis(beta-cyclodextrin) 1 and paclitaxel is discussed from thermodynamic viewpoint. Furthermore, the cytotoxicity of these complexes assessed using a human erythroleukemia K562 cell line indicates that the IC(50) value of 1/paclitaxel complex is 6.0 x 10(-10) mol/dm(3) (calculated as paclitaxel molar concentration), which means that the antitumor activity of 1/paclitaxel complex is better than that of parent paclitaxel (IC(50) value 9.8 x 10(-10) mol/dm(3)). This high antitumor activity, along with the satisfactory water solubility and high thermal stability of the 1/paclitaxel complex, will be potentially useful for its clinical application as a highly effective antitumor drug.