Objective To improve the method for data processing in the current biol. activity assay of interferon(IFN).Methods The parameters in four-parameter Logistic regression was constrained to make the curve"slope-proportioned"or paralleled.Results The exptl. data were analyzed by the improved method,and the results obtained were similar to those obtained by the method currently used.The improved method may be used for primary estimation of reliability of the assay as well as the confidential intervals of test results.Conclusion Constrained four-parameter Logistic regression may be used for the biol. activity assay of IFN,of which the model was more intact and reliable for statistical anal. than the current method for data processing.