Host defense mechanisms were studied in six patients with pemphigus vulgaris (PV) and six patients with bullous pemphigoid (BP). Polymorphonuclear (PMN) leukocyte killing of Staphylococcus organisms was evaluated, and chemotaxis of PMN and mononuclear (MN) leukocytes in patients was compared with that in twenty ageand sex-matched controls. All patients had extensive widespread disease with the clinical diagnosis confirmed by immunopathologic studies. No statistically significant differences were observed in the PMN leukocyte bactericidal activity between PV patients and controls. In BP patients, PMN leukocyte bactericidal activity was very slightly reduced when normal cells and patient serum were used, but activity was normal when patient cells and patient serum were used. PMN leukocyte chemotaxis was normal in PV and BP patients. MN leukocyte chemotaxis was normal in PV patients and increased in BP patients when compared with that in controls. This study indicated that in spite of very severe and extensive disease, patients with PV and BP have intact neutrophil and monocyte functions. Drugs that compromise the patient's ability to fight infections should be used cautiously and judiciously.