In this study, monoclonal antibodies (MABs) to human interferon alpha (HuIFN alpha) were evaluated for their suitability for the quantification of different HuIFN alpha preparations by conventional immunoassay methods. The results obtained using four different MABs were compared and suggest that whilst immunoassays can be highly sensitive and reproducible to perform, calibration of antigen content in relation to units of biological activity remains a problem, particularly because of the molecular and biological heterogeneity of HuIFN alpha preparations. Nevertheless, the judicious selection of MABs specific for an individual HuIFN alpha subtype has indicated that immunoassays may be accurately calibrated in units of biological activity for potency estimations of that HuIFN alpha subtype.