In this work the volumetric O-2 transfer coefficient (OTC) through a membrane of a miniaturized hollow-fibre bioreactor was measured by the use of modified O-2 microcoaxial needle electrodes. Before measurement, available electrodes were modified by inserting and glueing them in thin glass capillaries to avoid damage. No differences in the behaviour of the electrodes occurred in comparison to the non-modified electrodes. These modified electrodes allowed O-2 partial pressure measurement in the 0.8-mm-high extracapillary space (ECS) of the bioreactor with high sensitivity and reliability. O-2, measurements were carried out at the two ports of the ECS at different insertion depths. The results of the measurements showed a homogeneous O-2 supply during variation of the radial co-ordinate of the electrode. In addition to these results, an increase in the local supply in the direction of medium flow was observed. The calculated mean OTC (47-63 h(-1)) gave extremely improved O-2 transfer due to membrane aeration compared to conventional hollow-fibre systems and other bioreactors used in animal-cell culture technology. The improved OTC and the small ECS volume (4.3 ml) makes this culture system suitable for the cultivation of primary cells with tissue-like densities.