The effect of shear on the antigen binding activity of a recombinant scFv antibody fragment was investigated in the presence of air-liquid interfaces using a stirred vessel that was incompletely filled. Changes in binding activity of the scFv to its antigen were monitored using an optical biosensor which had been sensitized with hen egg lysozyme (the antigen). The biosensor response was used as a measure of scFv binding activity. In buffer solution (mean velocity gradient similar to 20,000 s(-1)), loss of binding activity followed a first-order model with a mean rate constant of 0.83 h(-1). In unstirred buffer solution, no such loss was observed. Similarly, in sheared fermentation broth there was no loss of binding activity and protective effects were attributed to the antifoam PPG.