Magnetic labeling by a magnetite-antibody conjugate has been combined with magnetic filtration (high gradient magnetic separation) to effect a rapid and efficient separation of a selected cell population from a suspension of single cells. Samples of more than 10(8) cells could be fractionated in about 5 min with complete recovery. The system has been applied to a model system using red blood cells (sheep or chicken) and commercial antibodies against species-determined cell surface antigens. Enrichments of labeled cells by factors of up to 37-fold were observed. The approach was relatively insensitive to details in the experimental protocol and to the number of unlabeled cells which were in the sample. Thus, the method was easy to use and can readily be scaled up to handle large samples containing 10(8) labeled cells in a total of 10(11) or more. It should be useful as a pre-enrichment scheme for suspensions in which cells of interest are rare and, consequently, very inefficiently sorted by fluorescence-activated instruments.