Amniotic fluid cells provide important information about the fetus. Studies of the surface ultrastructure of such cells identified four cell populations in mid-gestational fluids. A parallel study of tissue surfaces in contact with or communicating with the amniotic fluid showed that some tissues made no contribution to the cell populations, while others contributed extensively. Two principal cell shedding mechanisms were found in mid-trimester--exfoliation of whole cells and detachment of cellular fragments. The latter mechanism was observed from skin and umbilical cords during a limited period in mid-pregnancy. The majority of cells were derived from exfoliation of squamous epithelia.