MY-1 is the DNA fraction, isolated and purified from Mycobacterium bovis BCG, which is composed of water-soluble and heat-denatured nucleic acids. In preclinical study, MY-1 showed host-mediated antitumor activity against various kinds of syngeneic tumors, and revealed very low toxicity in animals. Since these results suggested that MY-1 could be used as a biological response modifier (BRM) for cancer therapy, we performed the phase I clinical study in patients with a variety of malignancies. Fifteen patients were treated using single s.c. injection of MY-1 at doses of 0.25-20 mg, and following 22 patients received 3-12.5 mg of MY-1 given s.c. 3 times a week for the period of 2 weeks. Mild and reversible side effects such as swelling, redness, and/or pain of injected site were observed in a few patients at dose level of 10 mg. There were no other toxic effects. In the latter 22 patients, immune parameters were measured weekly. After MY-1 treatment, enhanced PPD skin reaction and increased OKT4/OKT8 ratio in peripheral lymphocyte subsets were observed which were statistically significant especially at a dose level of 3 mg. The optimal dose and schedule for phase II clinical study were considered to be 3 mg s.c. 3 times a week. In addition, the methodology of the BRM phase I clinical study was discussed.