Type I interferons trigger diverse biological effects by binding a common receptor, composed of IFNAR1 and IFNAR2. Intriguingly, while the activation of an antiviral state is common to all cells, antiproliferative activity and apoptosis affect only part of the population, even when cells are stimulated with saturating interferon concentrations. Manipulating receptor expression by different small interfering RNA (siRNA) concentrations reduced the fraction of responsive cells independent of the interferon used, including a newly generated, extremely tight-binding variant. Reduced receptor numbers increased 50% effective concentrations (EC(50)s) for alpha interferon 2 (IFN-α2) but not for the tight-binding variant. A correlation between receptor numbers, STAT activation, and gene induction is observed. Our data suggest that for a given cell, the response is binary (+/-) and dependent on the stochastic expression levels of the receptors on an individual cell. A low number of receptors suffices for antiviral response and is thus a robust feature common to all cells. Conversely, a high number of receptors is required for antiproliferative activity, which allows for fine-tuning on a single-cell level.