Cryostat sections of fifteen full-term, normal, human placentae have been studied by indirect immunofluorescence, using characterized antisera to a large number of proteins, in order to localize particular protein antigens to well-defined anatomical regions. Actin, plasminogen, and transferrin were identified in trophoblasts. Beta-2-microglobulin was uniformly absent from these cells. The complement component C3, the immunoglobulin IgG, and fibrinogen and collagen were identified in trophoblastic basement membranes. Of these IgG was the most sparingly represented, and C3 was not found on all trophoblastic basement membranes. Many proteins were identified within the stroma of chorionic billi, collagen being the most abundant. Many of these persisted following prolonged washing of the tissues. All four IgG subclasses were present in stroma, IgG1 and IgG3 being most prominent. Stromal cells were positive for beta-2-microglobulin, suggesting a unique sequestration of this protein on stromal cells but not on trophoblasts. Placental stem vessels were surrounded by collagen, and the walls of these vessels stained brightly for myosin: the vascular endothelium contained beta-2-microglobulin and actin. Peri- and inter-villous fibrin were faintly positive for several proteins, but these areas stained intensely for fibrinogen, plasminogen, alpha-2-macroglobulin and C4. This approach has proven to be useful in establishing precise definitions of the molecular morphology of normal human placentae.