Recently we showed that internal electric fields for linear polyenes in n-alkane crystals could be determined by high-resolution optical hole burning spectroscopy in combination with a novel quantum mechanical approach to data analysis. Here we demonstrate that the same ideas can be applied to protoporphyrin LY substituted horse myoglobin to determine the two internal electric field components in the porphyrin plane. The orientation of the internal field shows that the dominant contribution is from the deprotonated propionic acid side chains of the porphyrin ring; the magnitude gives a measure of the contribution of microscopic heme pocket structure to the local field factor.