Ultrastructural changes in the prepubertal human prostatic epithelium maintained in vitro are described and are compared with the ultrastructure of the same tissue before culture. In cultured cells, rough endoplasmic reticulum and Golgi complex are poorly represented and the latter also loses its polarity. Increase in cytoplasmic microfilaments is discussed in relation to possible vitamin A deficiency. Primary and secondary lysosomes, arising from autophagy and endocytosis, occur in large numbers as autophagosomes, myelin figures, residual bodies, and multivesicular bodies. Prostatic acid phosphatase activity, an important secondary sex characteristic, is influenced by sex hormones and malignancy; since this enzyme is lysosome-associated, special emphasis is placed on lysosomal changes. Some ultrastructural changes in rough endoplasmic reticulum, Golgi complex, and the lysosomal system are similar to those observed after castration. This study presents ultrastructure of cultured cells which form the basis for studies involving neoplastic transformation, aging, and hormonal manipulation using an in vitro model. This is necessitated by the absence of an in vivo animal model for prostatic neoplasia; hence studies on prostatic oncogenesis, and age-related phenomenon, must be done on cells in vitro. Significance of this study is enhanced by the fact that normal human prepubertal prostate has not been studied before and normal viable prostate is generally not available for investigations.