Strong, Variable Circular Polarization in PKS 1519-273

We report strong variability in the circular and linear polarization of the intraday variable source PKS 1519-273. The circular polarization varies on a timescale of hours to days at frequencies between 1.4 and 8.6 GHz, and is strongly correlated with variations in the total intensity at 4.8 and 8.6 GHz. We argue that the variability is due to interstellar scintillation of a highly compact (15-35 \mu as) component of the source with -3.8\pm 0.4% circular polarization at 4.8 GHz. We find that no simple model for the circular polarization can account for both the high magnitude and the frequency dependence in PKS 1519-273 at centimeter wavelengths.