A new method for the determination of elastic constants of colloidal systems is described. We study super-paramagnetic microspheres confined by gravity to a two-dimensional layer at a water-air interface. Under an external vertical magnetic field the particles arrange in a crystalline triangular phase because of the repulsive dipole-dipole interaction. By use of an optical tweezer, one triangle formed by three spheres is rotated from its equilibrium position and the relaxation time measured using video-microscopy. We demonstrate that this time is directly related to the shear modulus µ of the crystal and study µ as a function of the magnetic particle interaction strength.During the last decades the interest in colloidal systems has grown tremendously because of their widespread technological applications and due to the availability of precisely calibrated particles used as model systems in "classical" condensed-matter physics [1]. The crystallization of colloids, both in two and three dimensions (2D and 3D), has been a continuous matter of particular interest. The research mostly focused on the analysis of structure and dynamics of colloidal systems on different length and time scales through static or dynamic light scattering techniques.Elastic constants of colloidal crystals -essentially the shear modulus µ-were determined from the shear-induced resonance of the crystal through light scattering techniques (see [2] for recent work). As in real crystals, the value of µ is found to depend strongly upon the crystalline morphology and changes significantly between randomly oriented crystallites and shear-ordered samples [3]. In addition, using this method, only a very reduced number of modes can be investigated. A different approach to determine elastic constants is based on the analysis of the thermally induced vibrations of the particles in the crystal [4]. These fluctuations are inversely proportional to the elastic constants but depend also upon the system size studied. Therefore a finite-size scaling method is applied to extrapolate to the macroscopic elastic constants. We will show [5] that this method is well applicable to our system as described below and gives results in good agreement with theoretical predictions. However, so far the method can only be used for defect-free samples, a situation not always easy to realize in experiments.