Carboxylic acid groups (CCOH) were introduced onto the walls or ends of the individual CNT/Fe3O4 composite nanofiber by sonication in a mixture of sulfuric acid/nitric acid. The carboxylated products exhibit favorable magnetic properties and excellent dispersibility in water, which are excellent for use in cell separation. it is found that human ascites cells with cancerization could internalize these carboxylated CNT/Fe3O4 composite nanofibers by endocytosis, resulting in successful magnetic separation. (C) 2008 Elsevier B.V. All rights reserved.