We used an enzyme-linked immunosorbent assay system to compare prevalence and levels of systemic and local antibody to Giardia lamblia in different populations. Serial blood and milk samples were collected simultaneously from lactating women in Texas and Mexico. IgG antibody to G. lamblia was present in 118 (77%) of 153 sera from 27 Mexican mothers but in only 51 (24%) of 214 sera obtained from 28 Texan mothers (P less than .001). Sera positive for IgG antibody to G. lamblia revealed antibody levels that were significantly higher in Mexico than in Texas (0.325 +/- 0.076 vs. 0.219 +/- 0.079 optical density units). Secretory IgA antibody to G. lamblia was found in 121 (79%) of 153 milk samples in the Mexican population, but in only 33 (15%) of 214 samples in the Texan population (P less than .001). The levels of secretory antibody to G. lamblia also were higher in the Mexican population. The study documents a widely different antibody response to G. lamblia in individuals living in different areas. As antibodies to G. lamblia can be acquired by infants transplacentally or through breast-feeding, this difference might be important in the relative protection against giardial infection.