A previously developed cell labelling methodology has been evaluated to assess its potential to precisely control the degree of magnetic labelling. The two-step method provides a quick way of labelling cells by first biotinylating the cell membrane proteins and then binding streptavidin paramagnetic particles onto the biotinylated proteins. Characterisation studies on biotinylated HeLa cells have revealed that the biotin concentration on the cell surface can be varied by changing the biotinylating reagent concentration. At the optimal concentration (750 microm), a substantial surface biotin density (approximately 10(8) biotin per cell) could be achieved within 30 min. The degree of magnetic labelling could be altered by adjusting the concentration of paramagnetic particles added to the cells and the binding of the particles onto the cell surface was not considerably affected by the biotin density on the cell surface. The magnetic moment of the labelled cells was measured and correlated well with the degree of magnetic labelling. Cell viability studies indicated that the magnetic labelling was not cytotoxic. Magnetically labelled cells were then successfully targeted and manipulated by magnetic fields to form three dimensional multicellular structures.