Photocopolymerizations of endo-3,6-epoxy-1,2,3,6-tetrahydrophthalimide (ETPI) with acrylic acid (AA), vinyl acetate (VAc), and maleic anhydride (MAH) were carried out in a mixed solvent of 2-butanone and acetone using 2,2-dimethoxy-2-phenylacetophenone as an initiator at 25 degrees C. Synthesized ETPI, poly(ETPI), poly(ETPI-co-AA), poly(ETPI-co-VAc), and poly(ETPI-co-MAH) were characterized by IR and H-1-NMR spectroscopies, elemental analysis, and gel permeation chromatography. The synthesized polymers have a number-average molecular weight ((M) over bar(n)) in the range of 3500-27,400. The in vitro cytotoxicities of poly(ETPI), poly(ETPI-co-AA), poly(ETPI-co-VAc), and poly(ETPI-co-MAH) against fibroblast and K-562 human leukemia cells were lower than that of monomeric ETPI at a low concentration (0.02 mg/mL). The in vivo antitumor activities of the polymers were evaluated by the survival time with sarcoma 180 tumor-bearing mice. The polymers showed higher antitumor activity and lower toxicity than both monomeric ETPI and 5-fluorouracil at all doses tested.