Rabbit antiserum to cultured trophozoites of Giardia intestinalis (G. lamblia) was used to detect organisms in jejunal biopsies using the PAP immunoperoxidase technique. In 30 sections examined from seven cases of giardiasis associated with histological changes in malabsorption, trophozoites were seen in the lamina propria in one instance, although they were otherwise seen in the intestinal lumen and surface mucus. One anti-Giardia serum reacted with the neutrophil polymorphs in all infected jejunal biopsies, and in jejunal and rectal biopsies from patients not suffering from giardiasis. The reaction with Giardia and with polymorphs could be absorbed out using washed Giardia trophozoites but not with preparations of bacteria.