The paper describes a recombinant Schneider 2 (rS2) cell culture and protein expression in a bioreactor. S2 cells were transfected with a plasmid containing a fusion protein (human mu opioid receptor, hMOR, and green fluorescent protein, EGFP) under the control of inducible metallothionein promoter. A bioprocess in a bioreactor with 5% dissolved oxygen, 27 degrees C and 120 rpm enabled the cell culture to attain 5.3x10(7 )viable cells/mL at 96 h. The induction decreased the cell multiplication (2.5x10(7) viable cells/mL at 72 h). Glutamine and glucose and low levels of lactate were consumed. A fast recombinant protein synthesis took place and, at 6 h of induction, 2x10(4) receptors/cell could be detected by a functional binding assay. Fluorescence measurements showed a progressive increase of recombinant protein expression with a maximal value of 1.26x10(5) fluo counts/s at 24 h of induction. The data shown in this paper indicate a practical and scaleable cell culture bioprocess procedure for the preparation of recombinant proteins expressed in S2 cells.