The interstitium, the connective tissue space outside the cells and the vascular and lymphatic system, provides mechanical and structural support within and between different tissues. This extracellular matrix has a central role in the control of cell proliferation, differentiation and migration. It is also the space where water and its dissolved constituents move from the blood plasma to the lymphatics. Analysis of interstitial fluid i.e. the fluid portion of the extracellular matrix, gives important information on fluid exchange and transport between the blood and tissue compartments. Interstitial fluid is, however, not readily accessible from tissues that are in the normal hydrated state. Lymph or fluid collected by implanted wicks have provided valuable information about the composition and dynamics of the most mobile fraction of the interstitial fluid in various organs and tissues, namely 'free fluid', the fraction most directly involved in interstitial fluid turnover. Still, interstitial fluid is inaccessible in several organs and tissues. We have successfully developed and refined a graded centrifugation method for isolation of interstitial fluid from various tissues, including the non-solid haematopoietic bone marrow, tumours and the dental pulp, and have thus been able to measure another determinant of fluid exchange in these tissues, namely interstitial fluid colloid osmotic pressure. Furthermore, we have shown a potential differential inflammatory response in tissue interstitial fluid and plasma. Our new approach has proved useful when studying transcapillary fluid balance and thereby fluid volume regulation and extracellular signalling substances under normal and pathological conditions in previously inaccessible organs.