Analysis of the neutralization of human interferon-α by the monoclonal antibody NK2 showed, by three different methods, that the avidity constant was about 1010 m-1. A small decrease in the avidity constant (and in the antibody titre) with increasing interferon concentrations was observed, suggesting that the antibody-interferon complex had a little biological activity. The antibody neutralizes the interferon by preventing the binding of interferon to susceptible cells. It does not react with human-γ, mouse-α, monkey, bovine or rabbit interferons.