Forty-two specimens of human prostatic neoplasia (32 carcinomas, eight benign hyperplasia, two bladder tumors infiltrating prostatic tissue, and 15 tissue cultures derived from prostatic neoplasia) were examined by electron microscopy. Intracisternal viruslike particles, 150-200 nm in diameter and budding, were found in epithelial cells of four carcinomas. In some of these particles, an electron-dense central core or two concentric layers were discernible. In addition, particles resembling type C virus particles, 90-130 nm in diameter, were observed in intracytoplasmic vacuoles in five cases of carcinomas and in one case of benign prostate hyperplasia. Thus, viruslike particles were found in 9 of 32 cases of prostate carcinoma and in one of eight cases of benign prostate hyperplasia. Virus particles have, so far, not been found in any of the tissue culture specimens. Further studies are required to determine the nature of these particles and their relationship to the origin of human prostatic neoplasia. Additional observations in both benign hyperplasia and carcinoma include intranuclear mitochondria, multilayered nuclear inclusions, bundles of intranuclear fibrils, intracytoplasmic tubules, extracellular tubulo-filamentous structures, and cilia.ltilayered nuclear inclusions, bundles of ilo-filamentous structures, and cilia.