Human decidual cells of early and late pregnancy were studied immunochemically and ultrastructurally with respect to the presence and nature of pericellular basement membrane material. The most prominent cell type in decidual tissue of both early and late pregnancy were large, mature epithelioid decidual cells (greater than 25 micron) with a distinct pericellular basement membrane, containing at least laminin, type IV collagen, heparan sulfate proteoglycan, and fibronectin. Laminin was extracted from the decidual tissue at a concentration of 3 micrograms/gm as measured by enzyme-linked immunosorbent assay. Biosynthesis of laminin was shown by [35S]methionine labeling of short term organ cultures of decidual tissue followed by immunoprecipation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and fluorography. The laminin chains migrated with the apparent molecular weights of 300 and 200 kilodaltons under reducing conditions. Two other separate populations of cells were apparent in the decidual tissue of early pregnancy. A smaller group of rounded intermediate sized (15 to 25 micron) decidual cells had focal deposits basement membrane immunoreactive material scattered at the cell surfaces. Ultrastructurally, clumps of electron dense basement membrane material were present in close vicinity to the cell membrane. Occasional populations of small (greater than 15 micron) elongated fibroblastic-like cells were identified which did not display any immunoreactivity for any of the investigated basement membrane components. The two latter populations of decidual cells were not observed in the decidual tissue investigated from the placental bed of late pregnancy. It is suggested that these three types of decidual cells might represent various stages of the sequential differentiation of stromal cells into decidual cells of the pregnant endometrium. Predecidualization of the human endometrium, which is seen in the late secretory phase of the normal menstrual cycle and in some states of hyperplasia, was also shown to be accompanied by the presence of deposits of laminin-positive material at the cell surfaces. In the latter case, these cells resembled the intermediate sized decidual cells of the pregnant endometrium. In conclusion, the results suggest that the process of decidualization and predecidualization can be characterized morphologically and immunochemically by the accumulation of basement membrane material in specific decidua cell subpopulations.