Monoclonal antibodies (MAbs) have different modes of action and toxicity profile compared to chemotherapeutics, which makes it interesting to combine these drugs. Addition of cytokines to MAb therapy may also augment immune effector functions utilized by MAb. In an effort to improve the therapeutic effect of a MAb-based regimen in colorectal carcinoma (CRC) patients, the effects of a combination of alpha-interferon (alpha-IFN), 5-fluorouracil (5-FU), granulocyte-macrophage colony-stimulating factor (GM-CSF) and mouse MAb17-1A was evaluated in 27 patients with metastatic disease. alpha-IFN was given s.c. once daily for 5 consecutive days and at days 4 and 5, 5-FU was administered as a daily i.v. bolus injection. After 2 days rest, GM-CSF was given s.c. once daily, days 8-14 and on day 10, MAb17-1A was given i.v. The treatment cycle was repeated every 4th week. One patient achieved a partial remission and 13 patients showed a minor response or stable disease >3 months, inducing an overall response rate of 54%. Responding patients survived significantly longer than non-responding patients (p=0.021). Median overall survival time for all patients was 75 weeks and progression-free survival time 15 weeks. Adverse events related to alpha-IFN, GM-CSF and 5-FU were as expected. The frequency of patients with an immediate-type allergic reaction (ITAR) against MAb17-1A at the 1st, 2nd, 3rd and 4th treatment cycles was 11%, 52%, 62% and 64% respectively. The planned MAb17-1A dose had to be reduced by repeated infusions. No patient received full dose of MAb17-1A from the 3rd cycle and onward. Compared to historical control patients treated with MAb17-1A alone, the present combination regimen seemed to improve the response rate (54% vs 15%) as well as progression-free survival (15 vs 7 weeks; p<0.05).