Increasing doses of MAb 17-1A (mouse IgG2A) have been given for therapy of patients with metastatic colorectal carcinoma (n = 28). Serum half-life (T beta 1/2) of MAb 17-1A after a single infusion was about 24 h. A constant serum level of MAb 17-1A could be maintained for a long time period by infusions every 2-3 days. Patients received 500 mg 3 days a week for 12 weeks (total dose 12 g). Side effects were mild and dose related but never required medical intervention except for three times (out of 243 infusions) when allergic reactions appeared. All patients developed IgM and IgG antibodies. Two patients experienced immune complex-related symptoms. Six out of 22 evaluable patients (27%) had objective evidence of tumor cell regression/lysis. One of these achieved a complete remission. Median survival for all patients was 12 months, for responding patients 19 months and for non-responding 11 months. 5/17 patients were studied for the development of anti-anti-idiotypic antibodies (Ab3). The presence of Ab3 in serum correlated favourably to the clinical outcome of the disease.