Magnetic poly(methyl methacrylate) (PMMA)/poly(methyl methacrylate-co-methacrylic acid) [P(MMA-MAA)] composite polymer latices were synthesized by two-stage soapless emulsion polymerization in the presence of magnetite (Fe3O4) ferrofluids. Different types and concentrations of fatty acids were reacted with the Fe3O4 particles, which were prepared by the coprecipitation of Fe(II) and Fe(III) salts to obtain stable Fe3O4 ferrofluids. The Fe3O4/polymer particles were monodisperse, and the composite polymer particle size was approximately 100 nm. The morphology of the magnetic composite polymer latex particles was a core-shell structure. The core was PMMA encapsulating Fe3O4 particles, and the shell was the P(MMA-MAA) copolymer. The carboxylic acid functional groups (COOH) of methacrYlic acid (MAA) were mostly distributed on the surface of the composite polymer latex particles. Antibodies (antihuman immunoglobulin G) were then chemically bound with COOH groups onto the surface of the magnetic core-shell composite latices through the medium of carbodiimide to form the antibody-coated magnetic latices (magnetic immunolatices). The MAA shell composition of the composite latex could be adjusted to control the number of COOH groups and thus the number of antibody molecules on the magnetic composite latex particles. With a magnetic sorting device, the magnetic immunolatices derived from the magnetic PMMA/P(MMA-MAA) core-shell composite polymer latex performed well in cell-separation experiments based on the antigen-antibody reaction. (c) 2005 Wiley Periodicals, Inc.