A cell for use in assaying antibodies to an extracellular ligand selected from the group consisting of a cytokine and a growth factor, which extracellular ligand initiates a ligand-specific signal at the nucleus of the cell, said cell comprising: (a) a first DNA construct having a sequence comprising a first set of one or more transcription control elements, said first set of one or more transcription control elements being inducible by the ligand, and a portion encoding a first reporter gene product, driven by said first set of one or more transcription control elements, which first reporter gene product can be detected when the first set of one or more transcription control elements is induced by the ligand, said first set of one or more transcription control elements comprising a transcription control element selected from the group consisting of interferon stimulatory response element (ISRE), gamma activated sequence, a NFκB recognition site, and a signal transducer and activator of transcription #5 (STAT5); and (b) a second DNA construct having a sequence comprising: (i) a second set of one or more transcription control elements different from said first set; (ii) a segment driven by said second set of one or more transcription control elements encoding a second reporter gene product, which second reporter gene product can be independently measured in the presence of said first reporter gene product, and vice versa; and, (iii) on a separate cistron, a segment encoding said ligand, also driven by said second set of one or more transcription control elements. (a) a first DNA construct having a sequence comprising a first set of one or more transcription control elements, said first set of one or more transcription control elements being inducible by the ligand, and a portion encoding a first reporter gene product, driven by said first set of one or more transcription control elements, which first reporter gene product can be detected when the first set of one or more transcription control elements is induced by the ligand, said first set of one or more transcription control elements comprising a transcription control element selected from the group consisting of interferon stimulatory response element (ISRE), gamma activated sequence, a NFκB recognition site, and a signal transducer and activator of transcription #5 (STAT5); and (b) a second DNA construct having a sequence comprising: (i) a second set of one or more transcription control elements different from said first set; (ii) a segment driven by said second set of one or more transcription control elements encoding a second reporter gene product, which second reporter gene product can be independently measured in the presence of said first reporter gene product, and vice versa; and, (iii) on a separate cistron, a segment encoding said ligand, also driven by said second set of one or more transcription control elements. (i) a second set of one or more transcription control elements different from said first set; (ii) a segment driven by said second set of one or more transcription control elements encoding a second reporter gene product, which second reporter gene product can be independently measured in the presence of said first reporter gene product, and vice versa; and, (iii) on a separate cistron, a segment encoding said ligand, also driven by said second set of one or more transcription control elements.