OBJECTIVE To investigate slow-release features of biodegradable anticancer 5-fluorouracil-loaded immunonanoparticles (5-FU INPs), and to assess their tumor cell killing activity in vitro.METHODS The method of vibrating dialysis at a constant temperature,and first-order derivative ultraviolet spectrophotometry were used to determine the drug-releasing character of 5-FU INPs. The methyl thiazolyl tetrazolium (MTT) colorimetric method was employed to assay the killing activity of 5-FU INPs on 5 tumor cell lines at different phases.RESULTS The 5-FU INPs had a favorable slow-release function, with a t1/2 release time of 10.4 days. The 5-FU INPs had a rather strong lethal effect on 5 tumor cell lines resulting in a positive correlativity between the killing activity and the action time and amount of the drug released.CONCLUSION The drug disposition is uniform from the 5-FU INPs,and there is no impact on efficacy of the 5-FU during preparation and degradation of the 5-FU INPs. The 5-FU INPs have a favorable function for drug release, and can maintain an effective killing activity over a long period of time.