AbstractIn this article, we describe a new approach that allows the determination of the magnitude of force required to break single plant roots. Roots were taken from transgenic tobacco plants, expressing a secreted monoclonal antibody. They were divided into four key developmental stages. A novel micromanipulation technique was used to pull to breakage, single tobacco roots in buffer in order to determine their breaking force. A characteristic uniform step‐wise increase in the force up to a peak force for breakage was observed. The mean breaking force and mean work done were 101mN and 97µJ per root respectively. However, there was a significant increase in breaking force from the youngest white roots to the oldest, dark red–brown roots. We speculate that this was due to increasing lignin deposition with root stage of development (shown by phloroglucinol staining). No significant differences between fresh root mass, original root length, or mean root diameter for any of the root categories were found, displaying their uniformity, which would be beneficial for bioprocessing. In addition, no significant difference in antibody yield from the different root categories was found. These data show that it is possible to characterise the force requirements for root breakage and should assist in the optimisation of recombinant protein extraction from these roots. Biotechnol. Bioeng. 2008;100: 803–809. © 2008 Wiley Periodicals, Inc.