In a phase I trial 34 patients with pancreatic cancer were treated with the murine monoclonal antibody (MAb) BW 494 (BI 51.011) directed against a glycoprotein antigen. The patients received repeated doses of MAb over a time period from 5 to 14 days (highest single dose 100 mg, highest cumulative dose 490 mg). During this treatment serum levels of murine lgG increased to 43.4 μg/ml. The serum half life of murine lgG ranged from 2 to 3 days. Repeated injections of MAb BW 494 were normally well-tolerated when given within the first 15 days. Two patients presented with fatigue and a neuritis-like syndrome 2 weeks after the last lgG infusion which had resolved spontaneously by the next day. Severe allergic reactions were observed in 3 patients after repeated injections of the MAb. These 3 patients had high levels of human anti-murine antibodies (HAMA). Four weeks after the first application of MAb BW 494, 17/18 patients presented with HAMA (lgG). It could be demonstrated that the anti-murine response was in part anti-idiotypic. At the moment 16/34 patients are eligible for evaluation of tumor response. There was no complete or partial remission; however, 2 patients responded with minor tumor regression up to 32 weeks documented by reduction of liver metastases and primary tumor in CAT scan. Five additional patients presented with a long period of stable disease after immunotherapy (up to 40 weeks). Nine patients had progressive tumor disease in spite of MAb treatment.