Human epithelial cells of the lshikawa endometrial line can be stimulated to differentiate and form multicellular structures in 4-5 day-old monolayer cultures by the addition of a protein factor from fetal bovine serum. Multicellular structures become obvious over an 18-30-h period as the cells enlarge, separate from the dish, and form domes. These structures are similar to those that result from polarization in other epithelial cell lines. lshikawa dome formation appears to be a multistage process. The appearance of enlarged differentiated cells is detected within hours of adding fetal bovine serum; these enlarged cells lift off the surface of the dish within 6-8 more hours. Domes are observed about 24 h after the addition of fetal bovine serum. Sometimes dome cells migrate into a "bud-like" structure that extends out from the dome. Differentiation of the domes is dependent on a factor from fetal calf serum that behaves similarly to a very large protein or complex of proteins, greater than 300 kd. Progesterone appears to enhance the formation of domes but does not elicit dome formation in the absence of serum factor. o 1995 WiIey-Liss, Inc.