Complete control of an electromagnetic wave requires access to its electric and magnetic vector components. Realizing this level of control with metamaterials has recently opened new avenues regarding negative refractive indices and invisibility cloaking. The required microscopic building blocks are artificial electric and magnetic dipoles. Magnetic dipoles oscillating at optical frequencies have become available only recently in the form of man-made split-ring resonators, essentially subwavelength resonant electromagnets. Previous experimental work has focused on arrays of electric and/or magnetic dipoles. For further developments in this field, knowledge of the properties of the individual dipoles is highly desirable. In this paper, using a modulation technique, we measure the absolute extinction cross-section of a single split-ring resonator for the first time. At the fundamental magnetic resonance, it is found to be about one-seventh of 2 at a wavelength of = 1.4 m, which is in excellent agreement with microscopic calculations.