Twelve adult patients with primary humoral immunodeficiency were treated for at least six months with IVIG 200 mg/kg/mo and then crossed over to a high dose of 600 mg/kg/mo. Polymorphonuclear and mononuclear cells of these patients were tested after the third infusion in the low-dose cycle and then after the third infusion in the high-dose cycle, each time a day before, four days after, and 14 days after intravenous infusion. Each time, patients' cells and normal cells were tested using normal sera and patients' sera. IVIG infusions led to a significant increase in the level of circulating IgG, which was much more prominent in the high-dose group. Phagocytosis, phagocytic index, intracellular bactericidal activity and chemotaxis of polymorphonuclear cells (PMNs) were at least as active as in healthy controls. Actually in both cycles patients' PMN's had slightly higher phagocytic activity than normal cells. Patients' serum in the high dose cycle supported chemotaxis better than normal serum. Efficient phagocytic activity was maintained throughout the cycle; however, it was more active (P < 0.0125) in the midcycle in the high-dose cycle. Superoxide generation was normal in all conditions. Monocytic function was also normal in all conditions tested. It may be concluded that as far as cellular phagocytic functions are concerned, the high dose of IVIG does not protect the host more efficiently than the low dose.