Forty patients with histologically confirmed neoplastic diseases were treated with the methanol extraction residue of Bacillus Calmette-Guérin (MER). Thirty-six received concomitant chemotherapy. MER was initially given intradermally twice a month, 1 week apart, at a dose of 200 mug into each of five sites draining different lymph node-bearing areas on the anterior body surface. Thirty-seven patients developed local ulcerations at least 0.5 cm in diameter at MER injection sites. Typical lesion evolution was characterized by erythema and induration followed by vesicle formation and central necrosis. Either granulation tissue or a thick nonulcerated eschar preceded healing, leaving a linear, flat scar. Systemic toxicity consisted of malaise, fever, and myalgias on the day of MER administration. No hematological or biochemical changes directly attributable to MER were observed. Dose titrations in decreasing 10-fold dilutions in a linear array in a single anatomical region were carried out on 35 occasions. All patients but three developed at least a 5-mm induration to the 1-mug dose within 2 weeks of titration. Dose reductions were necessary in 19 instances. The minimal dose that produced a 1-cm inflammatory lesion with central necrosis was 0.01 mug. Serial biopsies were performed. These indicated a time-related series of changes from a nonspecific inflammatory lesion to an acute inflammatory response with microabscesses, followed by noncaseating granulomata and ultimately fibrosis. MER is a quantifiable nonviable immunostimulant that obeys dose-response relationships in its cutaneous lesions.