beta-Lactoglobulin (BLG)-specific serum IgG antibody was measured by enzyme-linked immunosorbent assay in 1,392 serum samples from newborn to 5-year-old Ecuadorian children enrolled into a representative nutrition and health survey. At a 1:100 serum dilution, 62% of the children showed specific antibody (blank-corrected optical density greater than or equal to 0.1). This prevalence did not change with increasing age. More specifically, we did not observe a prevalence or titer increase of BLG-specific antibody in age groups where the majority of these Ecuadorian children experienced infection with rotavirus (8-24-month age groups) and enterotoxigenic Escherichia coli (8-12-month age group). In addition, BLG-specific antibody did not differ between children who did or did not experience an episode of diarrhea 15 days before blood sampling. We observed a small but statistically significant difference in BLG-specific antibody between subsamples of Ecuadorian children regularly or only occasionally ingesting milk. Titers were higher in the group consuming more milk.