Copolymerizations of N-glycinylmaleimide (GMI) with methacrylic acid (MA) and vinyl acetate (VAc) were carried out in 2-butanone using lauroylperoxide as an initiator at 70 degrees C. Synthesized GMI, poly(GMI-co-MA), and poly(GMI-co-VAc) were characterized by IR and H-1-NMR spectroscopies, elemental analysis, and gel permeation chromatography. The in vitro cytotoxicities of poly(GMI-co-MA) and poly(GMI-co-VAc) were evaluated using K-562 human leukemia cells and HeLa cells. From the cytotoxicity data against HeLa cells, the copolymers are less cytotoxic than monomeric GMI at dosage of 0.02, 1.0, and 5.0 mg/mL. Copolymers were very effective at any dosage tested. The in vivo antitumor activities of poly(GMI-co-MA) and poly(GMI-co-VAc) were also evaluated against mice bearing sarcoma 180. Monomeric GMI and its copolymers showed higher antitumor activity than 5-fluorouracil (5-FU) at any dosage tested.