Strong cationic polyethyleneimine (PEI) is used to trace anionic sites in lung, choroid plexus, glomeruli, blood vessels and also on collagen fibrils. In the mentioned tissues, PEI is found in the basement membranes with a spacing identical to that found on collagen fibrils. It is assumed that the acid endgroups of tropocollagen molecules are responsible for the binding of PEI.

Polyethyleneimine (PEI) is proposed as a tracer for use in electron microscopical investigations. Relative small molecules are available (molecular weight 600-60,000). PEI is soluble in water; it is not visible in the electron microscope without further treatment, but can easily be detected as a particle by contrastting it with phosphotungstic acid or OsO4. Using PEI of a molecular weight of 40,000, particles of 10 nm diameter can be produced. The strong cationic character of PEI results in electrostatical binding to anionic sites. Hence perfusion and immersion of tissues with PEI of various molecular weights offers possibilities to either study the location of anionic sites or pathways of transport. Anionic sites could be demonstrated in the normal and pathologic glomerular basement membrane. Work on the use of PEI as a marker particle in immunoelectronmicroscopy is in progress.