Oxygen mass transfer in clumps of Atropa belladonna hairy roots was investigated as a function of root density and external flow conditions. Convection was the dominant mechanism for mass transfer into root clumps 3.5 to 5.0 cm in diameter; Peclet numbers inside the clumps ranged from 1.4 x 10(3) to 7.1 X 10(4) for external superficial flow velocities between 0.4 and 1.4 cm s(-1) Local dissolved-oxygen levels and rates of oxygen uptake were measured in a flow chamber and in bubble column and stirred bioreactors. When air was used as oxygen source, intraclump dissolved-oxygen tensions ranged from 90% to 100% air saturation at high external flow velocity and low root density, to less than 20% air saturation in dense root clumps. Specific oxygen-uptake rate declined with increasing root density. When external boundary layers around individual roots were eliminated by forcing liquid through the clumps at superficial velocities between 0.2 and 1.0 cm s(-1), internal dissolved-oxygen tension was maintained at 95% to 100% air saturation and rate of oxygen uptake at 1.6 x 10(-6) g g(-1) s(-1) dry weight. Liquid culture of single A. belladonna hairy roots was used to investigate the effect of dissolved-oxygen tension on root growth and morphology. Total root length and number of root tips increased exponentially at oxygen tensions between 70% and 100% air saturation. Specific growth rate increased with oxygen tension up to 100% air saturation; this result demonstrates that hairy roots aerated without oxygen supplementation are likely to be oxygen-limited. No growth occurred at 50% air saturation. Growth of hairy roots proceeded with an average length per tip of about 1 cm; this value was essentially independent of dissolved-oxygen tension between 70% and 100% air saturation.