IgG of normal female ICR mice was obtained and covalently conjugated with an alkylating agent, melphalan. The conjugate exhibited marked antitumor activities against Sarcoma 180 and Erhlich carcinoma cells inoculated subcutaneously into ICR mice. Based upon this experiment, melphalan was conjugated with human IgG. Antitumor activities of the conjugate, named K18, were tested against a variety of murine tumor cell lines in vivo. Additionally, K18 was tested in vivo against MX-1, MK-2 and LC-10 human tumor cell lines (of the breast, stomach, and lung respectively). The results showed antineoplastic activities on both murine and human tumor cell lines, thus indicating a possible new antitumor agent.