Immunomagnetic colloids have the properties of solutions and, therefore, offer distinct advantages in their ability to bind to cells as compared to larger magnetic microspheres. B cells and T cells have been isolated with higher degrees of purity following incubation with monoclonal antibodies (MoAbs) and a goat anti-mouse Ig ferrofluid. This was demonstrated both with mixtures of cell lines and lines titrated into normal bone marrow. In all cases, low non-specific binding was observed. Maximal cell capture was obtained without washing out excess MoAb, resulting in cell separations that could be completed in under 30 min. The system permits not only the magnetic capture of cells onto pins in a high gradient separator, but also their recovery. Cells separated in this way appear to be highly viable and it is possible to manipulate them further as the immunocolloid is sufficiently small for it not to interfere with tests such as indirect immunofluorescence. The advantages and disadvantages of immunocolloids versus larger magnetic microspheres are discussed.