Corn dextrin preparations with average degradation degrees of 16.7 and 167, or average molecular masses of 2,700 and 27,000, were covalently introduced with 4.5 and 11.2 to 35.4 mol of phosphoric acid, respectively, per mole by dry-heating at 110 to 140 degrees C for 24 h with sodium phosphate, and their effects on cell proliferation and production of immunoglobulin A (IgA) were investigated in C3H/HeN mouse spleen cell cultures. All the phosphorylated dextrin had mitogenic effects and stimulated proliferative responses induced by lipopolysaccharide (LPS) and concanavalin A (ConA). The phosphorylated dextrin with an average degradation degree of 16.7 was divided into 3 fractions, which had 3.6, 4.0 and 6.5 mol of phosphoric acid per mole when the molecular mass of each dextrin was postulated to be 2,700. LPS-induced proliferation was greatest with the preparation having 3.6 mol of phosphoric acid per mole of dextrin, whereas ConA-induced proliferation was strongest with 6.5 mol of phosphoric acid. On the other hand, the divided dextrin enhanced production of IgA by approximately 1.2-, 2.0- and 2.2-fold in accordance with the increasing amount of phosphoric acid. These results indicate that phosphorylated dextrin stimulates humoral immune responses in mouse spleen cell cultures.