The performance of a horizontal rotating tubular bioreactor (HRTB) was investigated with a biological system under non-sterile conditions. A spontaneously developed microbial culture was cultivated in a simple glucose/yeast extract medium. A fermentative bioconversion was examined by different combinations of process parameters (bioreactor rotation speed 5-30 min(-1) and medium inflow rate 1-10 1 h(-1)). Bioconversion dynamics in HRTB was monitored by withdrawing the samples from five positions along the bioreactor. Investigation in HRTB showed a rapid and an efficient glucose conversion into different products of metabolism. Glucose consumption rate along the HRTB depended on medium inflow rate, while bioreactor rotation speed did not have a significant influence. Complete glucose conversion in HRTB was observed at inflow rates of up to 6.5 1 h(-1). The pH gradient along the HRTB was detected at higher medium inflow rates (6.5 and 10 1 h(-1)), but did not significantly influence substrate conversion efficiency. A discussion of its potential use and a comparison of HRTB with other bioreactors are also presented. (C) 2003 Elsevier Ltd. All rights reserved.