This paper presents a test for exogeneity of explanatory variables that minimizes the need for auxiliary assumptions that are not required by the definition of exogeneity. It concerns inference about a non-parametric function "g" that is identified by a conditional moment restriction involving instrumental variables (IV). A test of the hypothesis that "g" is the mean of a random variable "Y" conditional on a covariate "X" is developed that is not subject to the ill-posed inverse problem of non-parametric IV estimation. The test is consistent whenever "g" differs from "E"("Y"|"X") on a set of non-zero probability. The usefulness of this new exogeneity test is displayed through Monte Carlo experiments and an application to estimation of non-parametric consumer expansion paths.