Sections of human pulp tissue taken from the region of a carious exposure in four young patients each having a clinical history of spontaneous dental pain were examined with the electron microscope. All the tissues examined exhibited a generalized edema, and an infiltration with lymphocytes, plasma cells, polymorphonuclear leukocytes and macrophages. Varying degree of lysis of pulp and inflammatory cells were evident. The unmyelinated nerve axons of the pulp appeared to be least affected. Extracellular lysosomes were present in many of the necrotic areas. Edematous vacuoles were noted in two of the pulp specimens. In some of the sections these vacuoles appeared to be responsible for a physical distortion of adjacent unmyelinated nerve axons. It was postulated that such distortion may be a contributing factor to the pain of pulpitis. In another of the pulp specimens, micro-organisms were found intracellularly and extracellularly. Intracellularly they were present within the cytoplasm of polymorphonuclear leukocytes and macrophages and exhibited evidence of lysis. Only two morphological forms, a gram positive rod, and a gram positive coccus were identified.