The nuclear phosphoprotein encoded by the retinoblastoma gene (pRB) appears to play a central role in control of cell division and differentiation. It is generally accepted that pRB is ubiquitously expressed. We investigated the expression of pRB in normal human tissues using immunochemical techniques to determine the expression of pRB in specific cell types. Maturing cells, both proliferating and nonproliferating, rather than their progenitors possess the highest levels of pRB. Cells of stratified epithelia, such as those from cervix, display strong immunostaining in the nondividing maturing suprabasal layer, whereas basal cells showed low to undetectable levels of pRB. Similar patterns of expression were observed in simple epithelia and hematopoietic cells contained within distinguishable proliferating compartments and in germ cell development. These studies are crucial to our understanding of processes involved in control of differentiation (tumorigenesis) as well as tumor progression.