Carbohydrate and protein histochemical tests differentiated two major groups of mantle and pearl sac secretory cells in P. maxima. The first group contained basophilic mucous cells. One type secreted a neutral mucosubstance, another sulphated acid rnucosubstances. Four types of the latter were distinguished, mainly on differences in azurophilia, alcianophilia, and aldehyde fuchsin reactivity. Two types of acidophilic cells formed the second group. The first contained histochemically demonstrable carbohydrate and protein, the second protein only. Both types reacted in most of the protein tests and differences concerned only the degree of reaction. With the exception of one of the acidophilic cell types, cells were of localized distribution. The greatest variety of cells occurred in the marginal mantle. Nothing definite is established on the functions of the secretions but these appear to be manifold; they may also include participation in shell formation.