Basic problems in the quantitation of interferon (IFN) neutralization by antibody were considered. Commonly used experimental methods were examined, and the "constant antibody method" (the IFN dose-response [e.g., cytopathic effect] curve is depicted in the presence of fixed concentrations of antibody) was found to be the most convenient and informative; the "constant IFN method" which is widely used is in principle equivalent, but not suitable for tests of IFN of mixed antigenic types and for monoclonal antibodies. As previously proposed (J. IFN Res. 1, 61, 1980), the antibody titer is defined not by neutralization of a fixed absolute amount of IFN (such as 10 international units), but by the IFN titer reduction factor, namely as the reciprocal of antibody dilution that neutralizes 10 experimental units (EU)/ml of IFN to 1 EU/ml. This definition was found satisfactory for many antibodies to human and mouse IFNs, in that the value for given antibody did not change with changes in IFN sensitivity of the assay system, whereas the titer based on the absolute amount of IFN neutralized did. In other words, a given antibody will neutralize a larger amount of IFN when assayed in a system with a lower IFN sensitivity. A simplified model of neutralization reaction was described that provided a theoretical background for this observation. The model also allowed some inferences to be made about the relationship of the titer to the affinity and IFN-binding capacity of the antibody.