Infection of the gastrointestinal tract with the protozoan Giardia lamblia is increasingly recognized as a cause of gastrointestinal symptoms, including diarrhea and malabsorption (1–3). Indeed it was the most frequently identifiable cause of water-borne diarrhea in the United States in 1974 (4). Studies of jejunal morphology and function have shown a spectrum of change from normal mucosa with unaltered small bowel absorption to severe mucosal lesions with steatorrhea (1, 5). The mechanisms that protect man from infection with G. lamblia are largely unknown. Although some persons show spontaneous resolution of infection (6) others may show persistent symptoms for months or years (7). Reports of giardiasis in children with protein-calorie malnutrition (8, 9) and in adults after gastrectomy (5) suggest that susceptibility to infection might be influenced by nutritional factors and changes in gastrointestinal physiology. On the other hand, the importance of normal host immunity is suggested by the high frequency of giardiasis in persons with hypogamma-globulinemia (10, 11).