Patients receiving recombinant human interferon-beta 1a (IFN-beta 1a) produced in Chinese hamster ovary (CHO) cells were tested for the formation of neutralizing antibodies (NABs) to IFN-beta. Samples were tested in an enzyme-linked immunosorbent assay (ELISA), and if positive were then tested for neutralization of antiviral activity in an IFN-beta bioassay. A total of 793 patients with viral diseases, premalignant and malignant diseases, and multiple sclerosis received IFN-beta 1a in clinical studies. Long-term studies included 56 patients with cancer treated for 6 or 12 months and 334 patients with multiple sclerosis (MS) at the end of one year of treatment. All of the NAB-positive patients were found in the latter. Positivity in a single specimen was found in 14.4% of the MS patients. The incidence of sustained neutralizing antibody titres (i.e. positive in two tests at least 6 months apart) was 6.9% in this group. Comparison with results from other studies suggests that CHO-derived IFN-beta 1a induces less neutralizing antibody than IFN-beta 1b produced in E. coli.