Purpose Aggrecan is one of the major macromolecular components of the intervertebral disc (IVD) and its loss is an early sign of degeneration. Restoration of aggrecan, and hence of biomechanical properties, is a major objective of biological therapies. At present, assessment of aggrecan concentration via its glycosaminoglycan (GAG) content is accomplished using biochemical and histological methods which require sacrifice of tissue. A minimally invasive method for assessing GAG, and hence aggrecan, which can avoid destruction of tissue, would be of benefit. Methods We have developed a needle micro-osmometer that is capable of measuring flux of saline into excised human nucleus pulposus (NP) tissue. Using the isotropic osmotic stress technique to assess the swelling pressure of the excised NP tissue and assuming negligible collagen tensile stress, we were able to relate the flux to the tissue fixed charge density (FCD). GAG concentration is evaluated from its FCD via the radioactive tracer technique.Samples representing different ages (28-59 years) and degeneration grades (1-4) were analyzed. Results The flux is controlled by both the osmotic pressure difference across the probe's semi-permeable membrane and by the tissue permeability. A linear correlation was found between flux and the tissue FCD. The equation describing the linear fit is FCD/(total tissue hydration) = 1.97 9