This invention relates to a mutant trichosanthin (MTCS) protein and a process for preparing the same. The product of the present invention is largely reduced of antigenicity compared to native trichosanthin (TCS), while retaining the biological activities of native TCS, e.g., anti-tumor, anti-virus, abortifacient, and ribosome-inactivating protein activities. The MTCS according to the present invention is useful as a therapeutic agent of high potency and low toxicity for the treatment of cancer, AIDS, ectopic pregnancy and for inducing mid-term abortion. Low antigenicity allows safe multiples of MTCS.