The role of Giardia duodenalis surface molecules in the attachment of trophozoites to epithelial cells has been established through the dual strategies of characterizing G. duodenalis clones with deficient adhesion and blocking experiments with surface-specific monoclonal antibodies. Also, the infectivity of the analyzed clones was tested using Mongolian gerbils as experimental model. Two adhesion-deficient G. duodenalis clones, C6 and C7, were isolated from the wild type C5 clone which in turn was obtained from the WB strain. The adhesion efficiencies of C6 and C7 clones (48.2+/-4.9 and 32.6+/-2.4, respectively) were significantly lower as compared with WB strain or C5 clone (82.8+/-6.4 and 79.9+/-7.9). Analysis of radiolabel surface proteins by 1D and 2D SDS-PAGE revealed prominently labelled 28 and 88 kDa components in C6 and C7 clones and a major 200 kDa protein in the C5 clone and the WB strain. The 88 and 200 kDa components are acidic proteins by two-dimensional electrophoretic analyses. The most striking difference between wild-type and adhesion-deficient Giardia trophozoites was the reduced expression of a 200 kDa surface protein in the latter. Significantly, a mAb (IG3) specific for the 200 kDa protein that reacted with more than 99% of WB and C5 trophozoites and less than 1% of C6 and C7 trophozoites as determined by indirect immunofluorescence inhibited the adhesion of trophozoites from WB and C5 clone to Madin Darby Canine Kidney cells by 52% and 40.9%, respectively, suggesting a participation of this antigen in adherence. Finally, the functional relevance of trophozoite adhesion to epithelial cells was indicated by the reduced capacity of the adhesion-deficient clones to establish the infection in Mongolian gerbils.