Actin polymerization was studied as a function of temperature and in controlled thermal gradients. In the absence of a thermal gradient, we observe a polymerization transition with a temperature increase of less than 10K, and we verify that microrheology provides a local measure of the polymerized fraction. Both of these results are in good agreement with prior observations. Thermal gradients allow us to generate polymerization gradients. This polymerization gradient induces motion in embedded microspheres that is not seen in control samples.