Antineoplastic activity against human gastric and colon carcinoma cell lines was tested in eighty-three species of Korean plants including Korean medicinal plants which have been frequently used in oriental herb prescriptions. The plant materials were extracted with methanol and the cytotoxic activity was tested using a calorimetric tetrazolium assay (MTT assay). Twenty-six plant extracts against gastric carcinoma cell line, eighteen extracts against colon carcinoma cell line and fourteen plant extracts against both carcinoma cell lines showed antineoplastic activity at the concentration of less than $100{mu}g/ml$. The effective components from four species have been isolated and reported.