A novel way to test whether gravitropic sensing is intracellular or whole-cell based is to use media with a density higher than the cell. Experiments with high-density extracellular media indicate that gravity sensing in cytoplasmic streaming and in gravitaxis may use the mass of the entire cell. For example, in internodal cells of the green alga Chara, the polarity of cytoplasmic streaming reverses in media that have a density higher than the surrounding cytoplasm. In contrast, much evidence suggests that gravitropic sensing relies upon an intracellular mass, that of sedimenting amyloplasts ("statoliths"). Protonemata of the moss Ceratodon purpureus contain sedimented amyloplasts and are gravitropic. The effects of high-density solutions of iodixanol and bovine serum albumin (BSA) were tested on gravitropism in Ceratodon protonemata. Our data indicate that both iodixanol and BSA can penetrate the cell wall. All cells that were inverted or reoriented to the horizontal displayed distinct negative gravitropism in 10 to 60% iodixanol solutions with densities of 1.052 to 1.320, as well as in 10 to 50% BSA solutions with densities of 1.037 to 1.184 g× cm^-3. Cell density was estimated to range from 1.0044 to 1.085 g"cm^-3; these values are equivalent to the densities of iodixanol solutions of 1 to 16% and BSA solutions of 1 to 23%. Since protonemata continued to grow upward under conditions where the cells were lighter than the surrounding medium, gravitropic sensing probably utilizes an intracellular mass in moss protonemata. These data provide additional support for the hypothesis that amyloplasts that sediment are statoliths.