Western immunoblot analysis of aqueous extracts of feces obtained from five giardiasis patients and from experimentally infected gerbils (Meriones unguiculatus) with rabbit antiserum to Giardia lamblia cysts has revealed antigens of three molecular weight groups. A stepladderlike, evenly-spaced set of strongly reactive antigens (darkest at a molecular weight [m.w.] of 55,000 to 70,000) appeared in the gerbil feces from day 4 (first experiment) or day 2 (second experiment) and lasted to about day 7 but disappeared completely by day 8 and did not reappear later. These antigenic bands were seen in gerbils infected with two isolates of G. lamblia. These bands were not revealed when antiserum to trophozoites was used as the probe, nor were they evident in specimens from the patients or in a preparation of sonicated cysts. A second group of antigens, represented by two to three low-m.w. bands of approximately 15,000 to 20,000, was evident in both the blots of gerbil feces after approximately day 8 and the specimens from the giardiasis patients. The third group of antigens revealed by blotting experiments was a high-m.w. band (approximately 110,000) which appeared on a number of days (beginning of day 8 of gerbil infection), but this band was not seen in the human specimens. A clear band corresponding to the previously reported GSA-65 antigen was not seen in either the gerbil or the human samples. Some low- and high-m.w. bands were also detected by antitrophozoite serum in the gerbil samples, but these were weak and unimpressive compared with those visualized using anticyst serum. A monoclonal antibody-based antigen capture enzyme-linked immunosorbent assay revealed that Giardia spp.-specific stool antigen rose suddenly at day 3 of gerbil infection, at the time when fecal cyst numbers began to rise rapidly.