Molecular noise as a source of phenotypic variability in single cells has been reported recently in a number of biological systems as diverse as gene expression and signal transduction in prokaryotes and eukaryotes. However, measuring noise in a biological system has not yet allowed us to characterize the macroscopic cellular response observed at the population level. Using bacterial chemotaxis in E. coli as a model-system for signal transduction, we have established experimentally and theoretically the existence of a general relationship between behavioral variability of non-stimulated cells and the macroscopic cellular response to a small chemical stimulus. We find that one can infer the cellular response of stimulated cells from the noise in non‑stimulated cells; we subsequently show that the noisiest cells also exhibit the largest cellular response.