Non-specific reactions in enzyme linked immunosorbent assays for serum antibody to entamoeba histolytica and Giardia lamblia in non-endemic areas. Serum samples from 20 Indian children with diarrhoea were compared with those from 20 children resident in the United Kingdom who had been diagnosed as having ulcerative colitis, or Crohn's disease, or indeterminate colitis using enzyme linked immunosorbent assays specific for Entamoeba histolytica and Giardia lamblia. More than 50% of the United Kingdom patients had high IgG responses in ELISAs for E histolytica and G lamblia. A confirmatory ELISA showed that the British sera reacted specifically to bovine serum proteins rather than to protozoal antigens. Prior incubation of sera with 5% bovine serum prohibited this reaction. Bovine serum is an integral part of the crude soluble antigen used in most ELISAs for E histolytica and G lamblia and needs to be replaced with purified antigen preparations. The British sera also reacted to other commonly used blocking agents such as bovine serum albumin, casein, and normal sheep serum. These reactions were attributed to uptake of dietary antigens or an enhanced immunological response to these antigens in patients with inflammatory bowel disease. 